I looked up the Toneport UX2, and that is a really cool device.

With the Focusrite interfaces I think there's always an assumption that there's a computer doing the recording. I've seen people on boards complain about the Scarlet's lack of a "loopback", but I'm not entirely sure if that's the same thing you want or not.

It sounds like you want to run guitar in, and through effects/drive(in a computer I'm assuming?); then you want to be able to monitor the effects while recording it as just the clean signal?
When I use the 4th gen I'm usually monitoring the raw microphone input, but my software is capturing it with a noise-gate, compression, and envelope-filter. All of those are software effects in the computer, and I can route it any way I want it to be inside the computer.
I could definitely monitor it with or without effects while capturing it as clean in the software.

If you're trying to do it without a computer I'm not sure if it's possible-.
The Scarlet has a left and right output around back, and a single output on the front for the headphones. But I think it can only send the same signal to both.
So if you're monitoring clean in headphones the back outputs are the same thing except in left and right line-outs.

But yes, cars are typically 12v systems. Your transformer outlets I guess plug into the round 'cigarette lighter' port? I guess you would need an inverter to plug in a One-Spot.
I've known people who had a standard outlet in the back of their mini-vans, but I guess that's because it has a factory inverter.

And sure if you have any sort of questions I'd be glad to try and give you answers if I can.
I don't know how much of a tech nerd expert I am.
I've just always been a computer user, and tried different things with electronics. I've done the band/music thing too, but I'm no expert in that. Never really recorded anything but myself.

You mentioned metal guitar, and so I have to ask:
What kind of metal are you into? Metal is kind of a wide term.
I played in punk bands, and then in metalcore/grind type bands.
Converge is probably my favorite metal band, but their early sound is described as "hardcore punk with leftover Slayer riffs", but I like a lot of music. Even country :)