The option to boot with is a few entries below the one highlighted. When you boot it, you will be prompted if you really want to remove user-sysmaint-split. After confirmation, the system will automatically reboot and the “SYSMAINT Session” boot modes will be gone. I mentioned “once” to make it clear that you would not need to boot into this boot mode every time.
After that procedure is done, you can install Veracrypt normally from PERSISTENT Mode | USER Session.
Full disk encryption with LUKS is the most common option and is fully supported. For files that have to be moved around, they can be archived with LXQt Archiver and encrypted using OpenPGP. (KGpg, a recommended application that is not preinstalled on Kicksecure, can be used to do this. Click File > Open Editor in KGpg to get to the part of the UI allowing file encryption and decryption.)
I’m not aware of a good volume-based encryption app that works with Kicksecure out of the box. There is some work being done to find one, and a possibility that we could create one in the future. See:
I don’t use Mullvad VPN, so I don’t know. Can likely be confirmed by experimentation, or you could ask Mullvad’s support. (The only reason I know that the installation process should be the same as on Debian is because Kicksecure is based on Debian and anything possible on Debian should be possible on Kicksecure.)
I don’t know if Mullvad VPN comes with Mullvad Browser. I remember saying something to that effect once but IIRC that was a misunderstanding and I was corrected on that. It might come with it, it might not.
Kicksecure doesn’t recommend any particular default browser, since they all have flaws that prevent them from being a default choice. This is why browser-choice is provided, so users can pick whatever option they deem least bad from a pool of common, fully open-source browsers. Personally, I use Google Chrome since my threat model is able to tolerate its privacy intrusions and open-core nature, but I would never recommend it to anyone who’s trying to stay anonymous for obvious reasons.