Yes, if your snap is strictly confined it can only access serial ports on Ubuntu Core with a gadget that exposes them as required. You cannot access them from classic Ubuntu, or probably any other distro that doesn’t have a gadget.
Definitely. You’re talking about this:
https://kyrofa.com/posts/ros-production-obtaining-confined-access-to-the-turtlebot-4-5
As of now, I don’t believe further interface work on this (e.g. hotplugging) in planned. @niemeyer can tell you more. See this bug, as well as these topics for more details.