To me the English sentence is not a natural sentence in the present-tense form. When would an English speaker say that?! It is much more natural in the past:
We finished eating the apples.
So this is "we finish to eat the apples", literally? I guessed right but it's a little clunky for a native English speaker. Would be nice to have this syntactical rule spelled out.
Is "fin-" generic? Is it valid to say "Mi finlegas libron"?
That is a valid sentence.
Can you stick fin on the front of manĝi and say something like "Ni finmanĝi la pomojn"?
Why not "finigxas"?
That means that something has become finished
How would "Ni finiĝas manĝi la pomojn" be translated?
Not sure because since finiĝi means to come to an end; concluding; end; etc.. Maybe something like we ended eating apples, but I not exactly sure, though