Schmalle

HX Stomp can handle all your requirements: it has four inputs: L In, R In, L Return, R Return. No need for an external preamp (unless you use long cables between Stomp and the guitars on noisy stages and need buffers). With two TRS -> 2x TS split adapter cables you could use L+R In for one guitar, L+R Return for the other. Mixing the pickups can be done with the Pan block. Chosing the guitar can be done with chosing IN vs Return in the Input block (turn the lower knob). If you have one patch with the IN as input and another with Return then changing the patch is changing the guitar (unmutes one, mutes the other).

Notice that this only works as long as the R input is used. (It is mandatory to have a cable connected to the R socket). Alternatively the Return can be used instead of the R as a second input. You can select the Return(s) as input in the input block by turning the lower knob. Check whether Line or Inst (Global Settings -> Ins/Outs -> Return ) works better with your instrument.

No need to waste a precious split for that. Just use a simple Pan block.

Put a Pan block (Volume/Pan -> Stereo) first in the signal path and set it 100% left to select the L/Mono input only and 100% right for the R input only.

The logical conclusion of what is described in the OP's second paragraph is that the low level has nothing to do with the HX Stomp. It's at the mixer side.

The Dual IR block has a Delay parameter which you can use to phase align IRs. Set it neutral / to zero.

Cioks DC7 provides 6 W per outlet. You needs at the very least 10 W to operate. So you need a current doubler adapter cable and connect HX FX to two outlets.

In Command Center you can set up footswitches per preset. The CC Toggle Commands you're using offer Latch and Momentary as switching options.

Like @enjaltsuggested Command Center's Preset Command does what you want on the Stomp itself. So if you program a footswitch to reselect itselft you can then externally trigger that footswitch with MIDI CC#48+X where X is the footswitch number. A bit clunky but it works.

That behavior is selectable in the switching type option: latching vs. momentary.

There is no preset reload option for incoming PC messages. But there is a Reload option for snapshots in Global Settings -> Preferences -> Snapshot Reselect. With that you can send a MIDI CC#69 (Snapshot Select message) to restore a preset if you know what snapshot it is selected initially. This also works when you program your MIDI foot controller to send a combination of MIDI PC *preset number* followed by MIDI CC#69 *snapshot number*.

The Powerplant has a hint written on the side: "Max combined current for all 9V DC outputs: 500 mA". The HX Stomp consumes about 900 mA when running. So the Powerplant doesn't provide enough current / power for the Stomp.

Search for E Bow or Sustainiac - these are common solutions for getting infinite feedback without volume. That said you can generate an artificial feedback sound with predefinded pitch without even using the guitar. I attached a sloppy HX Stomp preset to give you a starting point. You don't even need the Feedbacker. Just use one or two 3 Note Oscillators feeding the amp. Make it footswitchable (Bypass Assign). Let it fade in with the Attack parameter. Add something like a Pitch Vibrato to let it become more alive. Go crazy and change the oscillator pitches via Contoller Assign or Snapshots. Be creative! FB Generator.hlx

Use the Four Cable Method [the manual explains it]. Add the Boss between the HX Out and the amps return jack.

"-28" is a boot failure and the comments suggest it's hardware related. Sometimes rebooting helps. Sometimes it's unresolvable without a hardware service. Open a support ticket.

MIDI is used to change stuff on outboard gear by sending messages. Those messages can be of several types. Most used are PC (Program Change) messages to trigger program (preset) changes and CC (Continous Control) to control parameters on other devices. Your picture shows a CC Toggle set up but for changing presets you need a PC command.

That cable should be fine. Cable from Helix MIDI Out to Boss MIDI In. Global Settings > MIDI/Tempo > MIDI Base Channel = OMNI (or the channel that the Boss is set to) Global Settings > MIDI/Tempo > MIDI PC Tx = ON Now changing presets on the Helix should trigger the same preset number on the Boss. Once that works you can set Global Settings > MIDI/Tempo > MIDI PC Tx to OFF and program each patch or snapshot or footswitch to perform a MIDI PC (program change) via Command Center.

Yes. The Paint Audio MIDI Captain DUO is compatible with the HX Stomp (XL) and you can program it to change presets using MIDI PC (program change) messages. Other stuff like controling effects or changing snapshots works, too.

The only way I can think of is using a Send block in front of each amp model. You'll then have to mix the two signals externally. Some amps have multiple inputs.

Are you using the fx in front of the amp (amp used as a pedal platform) or in the fx loop?

Make sure you haven't accidently set up a footswitch to change the preset in Command Center.

If by "CM patcher" you mean 4CM patches: These patches are made for the 4-cable method (read the manual's Quick Start section). You need a guitar amp for that. Without the amp's preamp in the loop you'll hear nothing. But you can bypass the fx loop block if you want to hear the effects on their own.

Name the mic for a meaningful answer.

Press MODE and TAP to bypass all effects. Manual p.11:

It depends. I'd like to eq my in-ears (Phones Out) exclusively with the Global EQ. But that's not possible right now. It makes sense to eq your amp to your needs - to add or cut some imperfect frequencies from the amp or to adjust to the room / space your playing in. It makes less sense to eq the PA sound with the Global EQ. That is what a mixer should do. Low and high cuts are in the cab and IR blocks. Adjust these in a rehearsal at volume in band context.