Metsis:

Altebrilas:Kiel is not a preposition, but a conjunction, and Prevo says:
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I know that kiel is a conjunction, but that does not help, since being a conjunction does not automatically imply nominative or accusative.

There is a whole chapter in PMEG of using kiel for denoting role, function or similar. I think this is one of the cases where Z got a brain fart and created a mess, by not having clear markings for what can be regarded as the essive (in the sense of "state of being"; als der Präsident) and translative ("something becomes something"; zum Präsident) grammatical cases.

What matters in Esperanto is the verb, what case government it takes. PMEG has an example and an explanation in that above mentioned chapter:

      Marta rigardis la parolantan virinon kiel orakolon. [Z] =
      Marta opiniis, ke la parolanta virino estas (simila al) orakolo. (...tiel, kiel oni rigardas orakolon.)

And PIV says the same: if you use konsideri in the sense of rigardi 6, then kiel + akuzativo.

The choice of "n" after "kiel" just depends whether the "like" comparison is about the subject or object of the verb:

• mi rigardis la serviston kiel princo (I looked at the servant like I was a prince, i.e. with superiority)
  
• mi rigardis la serviston kiel princon (I looked at the servant like he was a prince, i.e. as though he was greater than his station)

See the bullet points about "kiel honesta(n) homo(n)" (4 & 5) here: https://bertilow.com/pmeg/gramatiko/komparoj/kiel_...