It’s fine – unless you need very low latency (in which case you need a realtime kernel).
No, and you’re better off without JACK (unless you have a specific need for it).
Yes the installer asks, and can run apt-get.. for you. – Installing jack is the easiest way to setup real-time permissions for your user. So even if you don’t use jack, installing the jackd package is convenient.
Just like any debian sysem: sudo apt-get install jackd2
To change realtime-permissions later (in case jack was installed without those): sudo dpkg-reconfigure -p high jackd2 then re-login (or reboot).
It’s not too bad. For most plugin GUIs you will have to enable the openGL driver on the PI, but DSP works surprisingly well. Since it’s debian-based you can also get a lot of plugins: