Schmalle

It doesn't do stuff - I tested it. It's only reason of existence on path B seems to be to have an object to grab and relocate to path A.

When the mixer is in that path B position it doesn't do anything. Not sure why it is even shown.

Make sure Global Settings->Displays->LED Rings is set to Dim/Brt.

Commands are per Snapshot. In Command Center you'll find your answers.

How did you come to that conclusion?

A electronic device nearby could be the reason. Computers e.g. emit electromagnetic noise that your guitar will pick up. Try identifying the noise source using your guitar as a detector.

It's possible. You need to manually add the looper functions (Undo, Start/Stop etc) to the footswitches in Command Center. You then can add bypass or controller assigns for other blocks to those footswitches. The negative with this is that looper functions are always shown - you can't hide it like with the standard 6 Switch Looper Footswitch mode.

Check again because the Boss manual indicates that the CH SEL input is for latching switches.

Read the "Bypass Assign" chapter in the manual.

Yes, that is strange. Put Return L/R block as first block. It's default Mix is 50% which means that block should produce a signal consisting of 50% Main L/R and 50% Return L/R.

Set Global Settings -> Ins/Outs -> Return Type to Return. Put Return L/R block as first block. Every block after this will process the sum of both the main and the return inputs.

Enable Global Settings -> MIDI/Tempo -> Snapshot CC Send .

Excellent list - one thing to add: Helix' volume knob should be cranked to ensure unity gain.

Did you playback via USB 3/4 - re-amping the playback through the active patch?

Hillary.

If OnSong can send MIDI CC messages on the count quater notes you could send CC#64 (Tap Tempo).

Set Global Settings -> Footswitches -> Stomp Select to Press or Both.

A hum eliminator should do the trick - see @rd2rk's suggestion. If it starts humming when you close the ground loop with USB that doesn't mean you can't fix it by breaking the loop in the audio connection.

The HX Stomps power supply is rated 3A@9V. This is more than sufficient to power both units. You just need a split adapter cable.

On the device it is a different story. It won't show up in the list until your patch has enough DSP resereve.

Contact support and ask Line6 directly about this.

The feedbacker will appear as last in the Dynamics->Mono list unless the patch doesn't provide enough DSP reserve for it. Normally unusable items are greyed out in a list - but seemingly not if they are the last item. I'd call it a bug because it is inconsistent visibility of items based on their position in a list. And because once you free DSP resources so that the Feedbacker shows up in the list - it then gets greyed out if you again exceed DSP resevere.

High-pitched squeal + hum sounds like a ground issue. The HXFX gets connected to ground via the cable to the amp input (or return). Make sure the amp is grounded. Test the amp's GND continuity between the input ground and power cable ground with a multimeter. Check without the power conditioner. Check in a different room / power outlet. Check with a different amp.

Ratchet?!