Got it! I'm back on the road with a fully functional 24 speed hardtail. Polygon sent me a chain and 8 speed trigger to go with the wheel they mistakenly sent me.
I began with a 21 speed thinking I would replace the frame and rear wheel. I ended up replacing everything except the handle bars, fork and front wheel, front derailleur and trigger, crank set and seat. And I'll end up replacing the seat, since I've noticed its frame is tweaked.
I replaced the frame, rear wheel, rear derailleur, trigger and cables, chain and bottom bracket. The seat is leaning slightly to one side, so it'll be replaced as well.
Glad I stuck with it. Thanks to you all for walking me through it.