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 

my theory is that everyone is capable of singing but your ability to access and work with that vocal range starts with getting out of your head and operating from a heart centered, embodied place. and to do that requires training awareness and rewiring your operating system. 

you can’t sing from the head.