Hey, Liam.
Yeah that's a known issue and it comes up a bit with the nested/modal UI menus. I'm working on a systemic fix for that, it's unfortunately not as easy as it might seem on its face.
I haven't been able to reproduce the overlays coming back up after switching units - I'll take a closer look. This is on the Linux build, right?
Do you mean the tile tooltip, specifically for the Commander marine? It is a bit large. I might shunt it into a corner or screen edge, or make it toggleable like the overlays.
All units do have a sight range that can exceed their maximum targeting range and that's intended.
When an enemy is already targeted, the rotate camera hotkeys (Q and E by default) switch between subtargets.