Not a big thing, but I'm just wondering about the following sentence taken from Kellerman's 'A Complete Grammar of Esperanto'. Which preposition is preferable, 'en' or 'per'? Kellerman seems to be using them interchangably.

Oni tenas la kuleron en la dekstra mano. = One holds the spoon in the right hand.
Oni tenas la kuleron per la dekstra mano. = One holds the spoon with/by means of the right hand.

Both prepositions fit in the above sentence but they are different prepositions with different meanings. Just as a careful user would differentiate between 'kun' and 'per', so is 'en' a distinctively different word.

Also, these old courses, developed a hundred or more years ago, seem to use 'oni' much more than we use it today. Has the preference for 'oni' changed over time? I hope so. Comments?

In terms of preferability, I think that's really up to you and what it is you want to express or emphasize. From a grammatical point of view I think they are equally valid.

For example if I simply wanted to stress the means by which he held the spoon and little more, I'd say per. If I wanted to say something about the general way he was holding it, I'd use the "en" form as it creates a kind of image in the mind of the person's hand with a spoon in it. "Per" by itself might not be adequate for that kind of expression.

Also, these old courses, developed a hundred or more years ago, seem to use 'oni' much more than we use it today. Has the preference for 'oni' changed over time? I hope so. Comments?

Why do you hope so ?
I like using the word.

If you say 'per la mano', it's not necessarily in the hand, it could also be between his fingers. Anyway, the stress is more on the holding in my opinion. But if you say 'en la mano' the spoon is in the hand.