

For new users the main question is not what operating system but what window manager as that is what shapes day to day user interaction. KDE plasma is a solid choice.
Installing it on a separate drive should be no problem. Just select the correct drive during install.
I use F10 to get to the boot menu on drive and select the drive it needs to boot from there. I have used it once in the last year and although it required many updates its still working.
I lack credible sources but I assume in a similar case they just let you in and observe every move you make to make sure you don’t overreach the scope of your visit.
Now if you are known public speaker on democracy and are scheduled to speak at universities you may have more trouble but i imagine in this case you either never go there to begin with or at least carry a dummy phone for the time of your travel.