Yes, Paul did warn against philosophy and human tradition that would lead away from Christ.
Yes, in Acts 8, Simon selfishly thought he could purchase the ability of laying on hands to give the holy spirit, aka the gift of God with money.
Where does 2 Peter 1:20 say anything about Gnostics 'figuratively interpreting prophecy'? I do agree that the Gnostics were 'bothersome'

I think more in the thought process that Christ couldn't have come in the flesh because NOTHING good could come from the flesh which, I believe, is why John kept reiterating the fact the one must confess that Jesus Christ came in the flesh. (1 John & 2 John)
Yea, and there were the Saduccees who didn't believe in the resurrection . . . the early Christians always had people around who carried their own philosophies and traditions.
I believe that some transcribers, being men, transcribed with bias but I don't believe any of the references above had anything to do with the transcription of scripture.