Picture here:

Notice the "Mono" and "Stereo" there? Will there be a real difference in sound if I choose 1 over the other?

I've got a Double Muff & a Boss CH-1 and I have no idea how to use this feature on my amp (I don't have the manual to the amp anymore...). Please be detailed as possible.

*Bonus question: What is the Send/return feature even for?

thanks SO much!
the send and return is another name for effects loop.

run the send to the in of your pedals and the return to the out.

the only way it will make a difference using mono/stereo is if your using stereo (TRS) cables and your pedals operate in stereo.

just use the mono.
How to use a loop:
1. Plug a cable into the Send output.
2. Plug cable into pedal. Repeat until you have all pedals you want in the chain.
3. Plug last cable from the last pedal into the Return Input.
4. Profit.
No ??? is needed here because it is quite simple.

The Effects loop is a spot for you to put your effects after the amp's distortion. Things like Delay, Reverb, and Chorus/flange, and similar effects sound better when placed after distortion, so that is why that is there.
The cable that is plugged into the INPUT jack on your pedal needs to be plugged into the send jack on the amp. And the cable plugged into the OUTPUT jack should be plugged into the return jack on the amp. Use the Mono section.

But the muff really should straight into the amp. And chorus will sound fine just plugged straight in as well, but experiment and find what you like.