Jail breaking gives you root access so you can remove apps you don't want but doesn't stop any already made malicious code for the apps you leave on the phone. It also doesn't help you with malicious code in the OS itself.
When you install a new OS on a phone, it will replace the main OS but the wifi chip still has its own custom OS that is unaffected. Think of it like a stripped down OS that only handles communication. This is at least how it works on custom Android ROMs.
ADHD is a very real condition but it is probably over diagnosed. Something interesting to note. Getting a proper amount of social interaction can reduce problems with ADHD. It sets the mind in a learning mindset. Usually, school is designed so there is practically no social interaction. A few minutes between classes is not good enough for social interaction. More introverted people can't even use a small blocks of time at all so they practically get no social interaction.
As for a proper diagnosis. For older kids, it is usually done with an IQ test. A proper multi-hour one. For young kids, they have another type of multi-hour test.
If you want a Linux version for regular users then use Linux Mint Cinnamon. I would stay away from any Fedora based distros or distros that use the Gnome interface. Arch based distros is good if you want advance features but those are not distros for regular people. The only Arch based distro that I know of that is okay for regular people is SteamOS and that is mostly for playing Steam games and not a full computer interface.
The best phone I know of it a Pixel with GrapheneOS.