Can the average human train their voice to be capable of singing well enough to be ‘decent’? (not pro)

Yes. And moreover more people than not have perfect pitch when singing from memory https://news.ucsc.edu/2024/08/earworms-music-memory-perfect-pitch.html

The Farcaster formerly known as Kchamp. Books, beats, bodybuilding, bright ideas and beautiful things.

katherinechampagne.com 

I totally agree, we had an office Christmas carol last year and invited a voice coach surprisingly we all killed it
Just takes a little practice