Nearby is an adverb but can also be an adejective depending on how it is used. If it describes a verb then it is an adverb, ex. He fell nearby.
Yes, an adverb can modify another adverb. Example sentence: That was really truly the best movie ever.
"Considerably" is an adverb.
Yes it can
Yes. It can modify another adverb.Thanks for asking your question.Bye Bye now.