STOP!

Ever hear the saying that a camel is a horse designed by a committee?

IMO that's what's happening here. Too many people chiming in based on limited info, confusing the issue, and causing you to waste money.

Take the time to completely analyze the problem, and come up with a theory before making changes or spending another dime.

FWIW, I doubt it's the chain, or length of rear loop. As for your earlier reference to tension in that rear loop, that's not anything, since cable tension/housing compression are always equal, and determined by the RD's return spring.

For my money, I'd focus on cable friction, or something like a poorly cut end, a poorly seated ferrule, or something similar. Or poor action within the shifter.

But, don't go with my theory, because not having seen it, I can only guess along with everyone else here.

Seriously, find someone who knows his compost, maybe through a solid referral. Give him (her?) the bike with no info except "shifting is off" and let him work his magic unassisted. Answer questions, but don't offer any theories.

At this point, I suspect no parts are needed, except possible replacement of a butchered cable. However, if he suggests replacing the chain or whatever else has already been tried, let him know that.