In theory, yes, you could make a mess, and any firmware is supposed to be certified to allow the device to be used.
In practice, this has been a convenient excuse to keep a whole chip with a separate OS in every smartphone, and it is very difficult to isolate from the rest of the system (see Graphene OS efforts).
I say all firmware should be opensource. Whether you’re allowed to change them or not is a separate question… for now.
I’m sorry if that’s harsh, but my feedback would be: drop that chart!
It’s daunting, it’s going to freak out many newbies. Too much choice kills the choice.
You have one “default” at the bottom, Mint, so stick to that. Tell the newbies they can switch anytime to something else once they’re a bit more comfortable with the Linux-world. And if I’m not mistaken, you can install and try the main DEs with Mint also. Or you can recommend Ubuntu, or any other newbie friendly distro. Just pick one and don’t lose them over what they could see as an important difficult decision before they even get started.