The impression I got from the #1 echo:

-people who bought it just wanted a simple echo and therefor liked it
-people who didn’t buy it were completely uninterested in it because it had no features

I belong to the second group

technically you could just have a send/return loop switcher and put each channel of the SMMH (left and right) into a send/return loop. So you could record a loop in the left channel, then switch to the right channel and you wouldn’t hear it anymore. However the loop would still be going, so if you wanted to do a new loop, the length would be dictated by the ongoing loop.

mixer with an FX send is what most people do

Since you’re using Audacity though, it’s hard to get a good sound.

pretty much any 9v power supply will work

Here’s an idea: EH expression pedal with built in LFO. When you hit the switch (crybaby toe down style) it would switch from expression pedal to LFO mode. A rotary switch would switch from sample and hold, sine, triangle, square, and sawtooth. Then the expression pedal could be switched between controlling depth or speed.

That’s why I’d get a travel guitar.

Just put the right out to your mixer.

You could probably wire a volume trimpot right before the output.