Vocabulary as always, can be a largely trial and error process when writing.

I'll comment on the participle part, because what little I know about it is more than anything about the rest.

I've read up more on past participles some more and I still can't give a clear answer in this case.
The past active and adverbial participles are really abstract and strange to my English mind.

This case I'm gonna put my money on present adverbial, like you said. But I feel that a convoluted argument could be made for предпочтя. [past adverbial of the perfective counterpart]

Somewhere along the lines of
"I turned off the flashlight, having (decided I) preferred the dim light.", the 'decided I' part just implies the instantaneous act of realizing he preferred. (or I'm insane idk)

This is one of those verbs about opinion forming and impressions, like понравиться, where the perfective distinction is especially hard for non native speakers to grasp, unlike a verb like пить where most people can understand the concept of finishing a drink or not.

до верха <Edit: No I'm dumb. Ignore this.

I won't be able to do a remotely full job on the translation's corrections, because I don't know most of them, so I won't start on it