here's some follow-up!
Long story short, Sergei DID in fact manage to help me solve the backspace key-repeat (yay!!), but could not solve the all-other-key-repeats-broken issue, so we parted ways and he promised to forward the problem to some even HIGHER tier of Apple Management/Design/something-something, who got back to me weeks later and said essentially "Yeah we removed that feature on the M1 macs and later because you are the only person on planet earth who has ever used this function, ever." ¯_(ツ)_/¯