Input one is line or xlr. Input 2 is instrument or xlr, based on the button selection you mentioned.
Where you will get tripped up is in how pro audio works. A stereo signal is two mono signals, panned hard rt and lft. In home audio, stereo is one trs cable. I am not sure in your examples which part of the cable coming from the amp is carrying what.
For cables, and involving a mostly mono source like a guitar, there are 2 ways to connect. You can come out of the amp stereo via the 3.5mm, and then sum l+r to mono via the patching I referenced. Method 2 is to not sum, and just take one side of the l+r and trat it as mono. No real danger for a guitar, since both sides of the l+r have the same information. For a true stereo instrument like a keyboard, the l+r sides may be different based on the sound.