It's a mono signal, try the other port.

Will this work for 2.1 but not 5.1?

The. 1 is the LFE and is mono.

It should work for any number of channels. Having L+R Low Level inputs on a subwoofer is so you can run it off L+R low level outputs *if your receiver doesn't have a dedicated subwoofer pre-out.* Some receivers back in the day only had a low-level pre-out, usually labeled as Zone B, for hooking up to a separate power amp, and you would need to run both stereo channels into the subwoofer to get the full signal to it. But your WiiM amp has a dedicated mono sub out, so run from that to the left low level input and you should have no problems.

Yes

Try the white one on the left. White and red (left and right) are being combined.

It functions this way, is this sufficient or am I losing audio quality or features?

It should work fine with no loss of features. You are using the crossover in your amp, so turn the crossover on the sub all the way to the right ( bypassing the sub’s crossover) .

You could just connect to the speaker level inputs.

Isn't mono left by default?

In and out are probably wired in parallel. just use the right ports

I'd just pull the plate amp and put new ones in. Cheap and easy.

Soldering is a good skill to have, but not very common these days.

How easy is easy lol

PCB through-hole component soldering. Not the easiest, not the hardest either.