I guess it depends on the sub-genre of metal.
I have to say that for Metal in general, I have not found anything more versatile and awesome than the free CORPSE rig available on the Rig Exchange. It is by Deadlight Studio. We play all the 80s hair metal songs using this. It will also work for Metallica as well (just looking at your avatar
)
One thing I would advise though, is a mute pedal for your acoustic guitar
I was going to say that since this setup is for just the monitoring mix only, not FOH, it is probably not that big of a deal to lose control of the mixer for a short while. And yes, the worst part is missing the ability to mute or unmute something.
Now you need the mic to be live for that kazoo someone is playing, well, it's not coming back until the wifi recovers. 
Also, that distinction you mention is important. We are NOT talking about Internet access at all, just a private wifi (or wired) for the mixers to be able to be talked to.
I have an XR-18 at home that we have the Kemper hooked up to.
For the IP / tablet connection the recommendation is that even though wifi is supported on the mixer you don't want to rely on that alone. So get your own wifi router and make that part of your setup.
Use wired connections to the mixer and at least one computer that you can use to actually control it. The mixer is configured to use wired ethernet, NOT wifi. Then even if the wifi goes down (people complained about tons of cell phones trying to connect to the mixer wifi during the actual show making it go down) you'd have control over the mixer through the wired connection. Now if you have a X32 with the actual physical controls, I don't think it makes it that risky. That being said, a dedicated wifi router is probably more robust than the mixer built-in one.
So the tablets / phones to adjust the personal mixes would connect to the mixer through the wifi router, NOT directly to the mixer wifi access point.
(As for the audio connections I have the Kemper going to three inputs on the mixer, stereo out + DI. You can also use the very same laptop to record the whole show through USB from the mixer to the laptop.)
We have not heard a drastic tone change, however we listen to the Kemper through a mixer and PA speakers. So our set up is different enough that we just may have not noticed.
I believe that you can do this with any expression pedal.
Please read up on this in the manual. Also, there are tons of guys who use this daily, hopefully they'll chime in.
And just as a very quick test, change the loop config on the Kemper if you have the boost pedal in the loop to "Distortion". That probably will solve your immediate need.
Welcome to the board!
You don't need to use a boost pedal to achieve this, there are tons of options, check this thread out:
Edit: Why I am recommending reading materials You may want to check this out, if you put a gain-type pedal in the loop, may want to switch to the Distortion loop type:
Drive into input = amazing, same pedal into loop, set in stomps section = sucks
Yes, we have one basically permanently hooked up in the X slot stereo loop. Works really well and it is very noticeable. We have it set to 1 Dub, both Effect and Dry cranked and the Tightness you'll hear how much you like.
The one thing is that the slight "jitter" that it adds to the 2nd guitar dub can be a little bit annoying when you want to keep a fast rhythm. We just bypass it during those times.
A buddy of mine told me about Babymetal and while I still cannot believe that it is real and someone mashed thrash metal and J-pop together, I could not help but notice the double rack setup at 2:22 in this video:
Even though it looks real, there is a chance that the whole video is photoshopped, size of the audience, makeup of the musicians, the Kempers themselves
Yes, Ratt was great! I am surprised you did not mention Round and Round, which was featured in The Wrestler, which - to me - happens to be the 2nd best Mickey Rourke movie after Angel Heart.
If we are on the subject, I have to say that lately I am listening to L.A. Guns more and I really think they were pretty underappreciated back in their time. Just like you said with Lay It Down, you hear this and after 2 bars you know it is awesome non-apologetic 80s music:
balazs respect on the 80s and the Terminator. The kids make jokes about how many times I saw that (and T2 as well). There was a time when I think I could recite basically the whole movie
Great song btw, I now want to play it.
Great idea, I would buy one immediately if available.
The competition is already there though, as we discussed here a lot, check out the Line6 HX Stomp. Small, light, can go straight to PA.
I want that just with Kemper profiles. Hook the Kemper Stomp to the computer, load 10-20 profiles (with effects). Any configuration can be a menu diving one with very few knobs / buttons because people will not do it a lot.
Edit: Just looked at the HX Stomp manual, very interesting, they show it as a "DSP Expansion" hooked up to a Kemper on page 9:
I hope someone who uses the same interface will chime in.
Looked through your pictures again, I don't see anything wrong. If you switch that 3rd channel from output 3 to 1 or 2 and play channel 3, you can hear the DI signal in your monitors, right?
Where does the reamping process break?
I assume that you have the DI signal recorded all good. (Not that it matters but I think you can use the front input for the guitar when recording DI, don't need to use the alt input on the back.)
I am not sure about your interface and DAW config. I believe that your Kemper input config for reamp is correct. When you hit play in the DAW, can you see the input LED blink on the Kemper?
If yes, can you hear the reamped signal through headphones?
I would say, if you have to use headphones (max volume constraint in the middle of the night, or just don't have monitors, FRFR or a PA), just plug in whatever you already have.... and enjoy what you'll hear. You can worry about buying something else that sounds better later. I have plugged in everything into our Kemper by now for headphones that we have at home and they all sounded great - even the SkullCandy (spelling?) that did not have its "bass booster" battery changed in 10 years.
My recommendation would be though that if you want to tweak profiles for live use, do not use headphones. Use as close to the real setup as you can when getting profiles ready to rock live.
Initially I got one Behringer Truth B2030 for the Kemper for a similar reason. If I don't want to turn on the whole system, just use that. It worked great.
In fact I like the speaker so much, I got another one and now use both in a different setup all the time. Inexpensive ($130 each), pretty substantial in weight and even in size compared to the little desk monitors. They sound great though. I recommend you check them out.
Alternatively, if you plugged it in backwards, like we all did once, you could try to sing into the headphones and see if the guitar plays the tune! 
I have only seen something like this a few times when trying to go through rigs very very fast. Like press the up-down-left-right button as fast as you can and I did get the everything is hanging and even the chicken head won't turn the box off. Like you said, it does power down after a normal power down period, just does not show it on the screen.
Since then we just got a little more careful with the rig change and wait ~1 second between changing rigs. Have not seen the issue since.
Do both speakers have the same impedance curve?
You may have said it already and I just missed it but does it sound better with just one of them hooked up?
I don't think there is anyone who does electronic anything with speakers who has not connected a speaker to a non-amplified source.
I know I did .... multiple times So welcome to the club.
