1) Regarding adverbs, can you make any adjective into an adverb using e as a prefix? Like end-onest for "honestly/honnêtement"?
No, actually I can't think of any adverb that would be formed like that in natural Breton. I think there were a few examples in Medieval Breton, and learners now use that prefix very often, to translate easily the French adverbs. Normally, in native Breton you use the adjective as an adverb, or you say "ba ur feson" (in a way...) + your adjective...
2) And I hope these sentences make sense:
N'em eus ket dispignet arc'hant nʼeus forzh hiziv.
I didn't spend any money at all today.
n'eus forzh means "it doesn't matter".
Your sentence would be "Ne'm eus ket dispignet tamm arc'hant ebet hiziv".
(Also, is there difference between n'em eus, ne'm eus and ne m'eus?)
ne m'eus is wrong. In written Breton, some write "n'em eus" but historically, the right spelling is "ne'm eus", since "em" is composed of the particle "e" + 'm eus. When you say "ne'm eus", the first "e" is a part of the negative particle "ne", it isn't the particle "e", since you don't use the particles "ne" and "e" at the same time
Gwiskañ a vez lunedoù met n'emañ ket o wiskañ int bremañ.
He wears glasses but he is not wearing them right now.
I wouldn't use "gwiskañ" to talk about glasses, you rather use it to say "to put...on". "To wear" is rather the verb "to be" plus "gant" ("a shirt is with me").
So I would say: Lunedoù en deus met n'emaint ket gantañ bremañ" (he has glasses but they aren't with him now).