Yes, you can install on one machine, move the drive to a different machine, and it should mostly just work (unless they are different architectures and the packages aren’t compatible). I’ve recently moved an old Arch install into a basically new computer and after changing the microcode it just works fine.

Yeah I think so. There are some optimisations that can be done to improve things, but at the end of the day it’s running off a slow USB drive

I’ve just got Arch Linux installed on a USB drive. I set it up so it works with both UEFI and legacy BIOS boot. It works great - I can install whatever tools I need on it, I can even use it as a little portable Linux drive if I need that. I’ve also used it to install Arch on a number of my computers.

The only issue is that the USB drive is pretty slow, so it’s not the best experience using it

I doubt it - the bootloader is very different on iPads and I don’t think it would let you boot a different OS

Only if the hash function is designed well

For a launcher, I’ve been using Neo Launcher (which is on IzzyOnDroid). It’s not as feature full as Nova but it’s pretty similar in spirit and style