I experimented with "na" and "far" as a beginner. I thought "the language evolves, so there's no problem with using these new forms, it's just the language evolving"

The difference between 'na' and 'far' is that the case for 'na' is weak, and its adoption would bring in complications as to when it was to be used (to say nothing of its ugliness, and that it largely remains just a drawing board proposal).

On the other hand the usage of 'far' has a long history and 'far' is actually useful/elegant as a shorter version of the longer 'fare de'.

Obviously la traduko de la libro de Smith is capable of more than one interpretation - the translation of Smith's book, or the translation by Smith.

Recourse to 'fare de' or 'far' will disambiguate.

Whilst the introduction of new (lexical) preposition requires no re-writing of earlier texts, the adoption of 'na' would tend to invalidate earlier literature - particularly if the rule of use was for all instances where the accusative marker would be absent in classical Esperanto.

And it is difficult see why if you had to say mi admiras na Zamenof, you wouldn't also have to say, to be consistent, mi havas na iom da mono, or indeed mi nun komprenas na kio estas la problemo.

I think that Esperanto is sometimes presented in a slightly idealistic and inaccurate way to beginners. "It has flexible word order, so people from different linguistic backgrounds can use the order that is most comfortable to them!" is a statement in that vein. It's true that Esperanto has free word order but people don't actually vary it willy nilly, and people don't generally stick with the word order of their native language. Experienced speakers all eventually learn, like Eltwish says, that Esperanto word order is flexible and it may vary to add emphasis, clarity, or a poetic sound, but the default generic order is SVO, regardless of the background of the speaker. Therefore a statement such as "John batis Steve", in the absence of all other markers, is easily understood by everyone as indicating that John hits Steve.

Interestingly, when I used to correct lessons, I found that people did NOT generally do word order according to their native language. However, if it was, say, an English speaker who had studied Spanish or German, the English speaker would start to write Esperanto sentences as if they were Spanish or German (adjective after noun, or SOV, for example). It has been so rare for me to see an actual native speaker of Spanish or German do that.

Although my mother tongue is SVO/SOV, in Esperanto I heavily use also other word orders in order to mark something or because I forgot something in the beginning of the sentence (BTW: adjective after noun is handy in this regard, too).
For me a sentence like "John batis Fred" is totally ambiguous without context, and having to cope with such sentences quite often (because I don't speak E-o in some club evening but with my son about real people and objects having non-esperantized names) I try to avoid such ambiguities using a simple, elegant, and laŭfundamenta way: "John batis je Fred"/"Je John batis Fred".

Kirilo81:I try to avoid such ambiguities using a simple, elegant, and laŭfundamenta way: "John batis je Fred"/"Je John batis Fred".

This is a great answer and I think what OP was looking for. Generally, you'd want to use the accusative here if possible because we have a direct object, but there is nothing wrong with using "je." I appreciate how "na" was made for this specific purpose, but it is unofficial. "Je" is a perfectly good alternative because it's a legitimate pronoun.

I don't tend to use "je", it sounds unnatural to me.

If I feel for some reason that it could be unclear, I just add -on. "John batis Fredon". An addition of another noun would be another way to do it, if you didn't like the existing ways. "John batis la malbonulon Fred"; "John batis sian amikon, Fred".

It's relatively rare though, to have a simple SVO sentence that is formatted as [proper name][verb][proper name], with no explanatory context whatesoever. I have used Esperanto in a lot of home and work situations (long-term Esperanto relationship in the past, plus present-day Esperanto work on websites and other activities) and I haven't had a problem with this. And if someone's name ends with a vowel sound, I just add -n, regardless of whether it is a normal Esperanto ending ("Mi vidis Benji-n" [pron. benĝin])

Kirilo, I have never heard any Esperantist (experienced or otherwise) use 'je' in the way you suggest. In fact if I heard 'John batis je Fred' I would imagine that he/she is trying to convey some other meaning than John batis Fred.

Perhaps, commenting on some fight contest, that John was matched against Fred, or that John approached Fred with arms flailing.

Compare John manĝis Fred with John manĝis je Fred. The first is an act of pure cannibalism, but the second would not imply that Fred was eaten. Perhaps a meal was eaten in Fred's honour or on his expense account.

Ni trinku je Fred, would certainly not naturally imply that Fred was a liquid.

but there is nothing wrong with using "je."

Agreed, if you mean something different

Yes, using je as expression for the direct object is innovatory, but it conforms with the function of je, makes the troublesome na unnecessary and is in most contexts clear.
You could make three objections to the use, but none is cogent:
1) The rule is "the accusative can replace je", not the other way around.
2) Je is too vague for the expression of the accusative.
3) It's uncommon.

Basic assumption: Je has the dictionary meaning "in some relation to".

ad 1): Je is a preposition. Prepositions express relations, as do cases. It's no accident that what E-o expresses per de and al are cases in other languages. The -n in E-o does express various relations, too, which can partly be expressed by prepositions. So there is no natural boundary between relations expressed by cases and those expressed by prepositions, and there is no linguistic reason not to express an object (which is a "wrap expression" by itself, compare the different "objects" of beat - kiss - see - like) with a preposition and no such prohibition in the Fundamento.

ad 2): The lexical vagueness of je makes it suitable for contexts, where other prepositions/the accusative don't fit, but pragmatically it is in any case more specific, or would someone understand e.g. Ŝi estas graveda je filo as "She was made pregnant by her son" or Ni trinku je Fred "We drink Fred's blood"? The lexical meaning does allow such interpretations, but most combinations involving je have long been conventionalized.
The rule is simple (and universal): Use the more specific item, if possible (and at no additional cost)*. So if there is the possibility the use a specific preposition/the accusative, the use of je would be inappropriate and one would search for another pragmatic interpretation. But if there is -for semantic or formal reasons- no fitting preposition/accusative, je should always be a viable solution, unless it this would result in a combination, where je already has a specific meaning in combination with a transitive verb (I don't know any example except from trinki).

*E.g.: We always use ŝi talking about women, because it is more specific and equally complex as the more general li/ĝi, but we don't always use -in- to mark womens professions etc.

3) I recognize that. As je is lexically vague, it can be puzzling to see the relation when first encountered, but if the use catches on as an elegant replacement of na, one will get used to it. It didn't take me long time to automatize this use of je in my idiolect.

Kirilo81:Although my mother tongue is SVO/SOV, in Esperanto I heavily use also other word orders in order to mark something or because I forgot something in the beginning of the sentence (BTW: adjective after noun is handy in this regard, too).
For me a sentence like "John batis Fred" is totally ambiguous without context, and having to cope with such sentences quite often

Yeah I'm the same way. Quite often my word order will depend on how careful I'm being, and usually I'm not that careful. If anything, the word order seems much more free in spoken Esperanto than written Esperanto, where non-SVO orders look stilted even though they're not incorrect.

I try to avoid such ambiguities using a simple, elegant, and laŭfundamenta way: "John batis je Fred"/"Je John batis Fred".

Interesting. I don't think I've ever encountered the situation in spoken Esperanto, but if I did I would probably just resolve it with -on or "John estis batita de Fred". Actually I'd probably say "John batiĝis de Fred", knowing it was incorrect but that I would be understood.

Yes, using je as expression for the direct object is innovatory, but it conforms with the function of je, makes the troublesome na unnecessary and is in most contexts clear.

For the use of 'je' to be clear, to flag a direct object when it is inconvenient to flag with '-n', it is necessary that the unflagged form be unclear.

But as has been pointed out in this thread already, the default SVO order makes sentences like John batis Fred clear anyway. So the average Esperantist is bound to see the use of 'je' in such sentences as carrying a different meaning.

So John amas je Susan will be seen as something other than John amas Susan, just as Ni trinku je Fred doesn't mean Ni trinku Fred.

Where is it going to end? Would you say John havas je iom da mono, or John nun komprenas je kio estas la problemo.

sudanglo:But as has been pointed out in this thread already, the default SVO order makes sentences like John batis Fred clear anyway.

There may be a bit of a bias here in an English forum, don't you think?

sudanglo:Where is it going to end? Would you say John havas je iom da mono, or John nun komprenas je kio estas la problemo.

From the PIV entry on je:

5 ĉe mezur-adjekto, kiam la mezursuplemento estas plurvorta: vi estas certe je duono da kapo pli alta ol miZ; mi estas je unu jaro pli juna, k tamen je kiom mi estas pli saĝa ol viZ; ŝi retiriĝis je kelke da paŝojZ; ili estas malproksimaj unu de la alia je tritaga vojaĝo; ĝi restis je granda distanco malantaŭenZ; ĉu la ombro iru antaŭen je dek gradoj?X; (analoge) je la unua fojo ŝi sentis larmojnZ