Mint is a long term support distro using an in house custom desktop environment. LTS distros don’t receive kernel or mesa updates as often so game performance can be lacking especially for newer games and/or newer hardware. Cinnamon also lags behind on modern desktop features compared to GNOME or KDE Plasma.
Signal has been pretty throughly audited by data security experts. It’s as secure or more so than Telegram. It uses end to end encryption, same as Telegram. If you’re crossing the border, unlink your device, delete the app, and relink it later. Your account can’t be restore via SMS. I’m not sure what you mean by that. I’m sure my government can collect any data they want if they’re determined enough, but Signal is about as secure as it gets if you’re talking civilian digital communication.
I’ll take your word for it, but I never use any other browser so I’m happily ignorant by continuing to use Firefox. A few times I’ve needed to open Edge since it’s already preinstalled on Windows. The few times I’ve done so I’ve not noticed any significant increase in performance. On the contrary, the user experience was pretty horrible.
This still happens with iPhones even when they’re powered down I believe.