re 6) I guess a configurable toolbar/menu system would be a good start then. Some extensions (eg Scale Tools) are hidden under pointless submenus. Worse, some extensions rely on a 'program within a program' just to access simple tools which could be done with a keystroke.
re 7) Using Scale tools as an example; If it became so popular that virtually every pro user installed it, and Trimble decided it should be added into Sketchup Pro (along with extra features eg"scale by reference"?), would they just go ahead and do that? I'd hope so.
re 8). Definitely! Constraining drawings to 'planes' or levels would only be an option, as it is primarily used in building, structure design (and documentation) only. I wouldnt mess with SU's natural modelling process. A Levels-like outcome can be acheived with a combo of 'invisible gluing planes" , sections, layers, scenes and Layout views... but a "Levels tool" could be more powerful and far easier. A really good extension could do the job (but then we have yet another workflow created around a program built by 1 person who may not care to provide ongoing support).
As an aside, I guess my Feature requestes depend on what SketchUp is trying to be? A professional design package for buildings, structures, civil works, etc? ...or to just stay as a "quick easy modelling tool to play around with concept designs"? I'm using Sketchup to do a similar job to Revit/etc and am often able to do better work than others can with $80k worth of "professional" BIM software, but....my gosh it's a lot of trial and error (14 years) figuring out workarounds to things because certain core functions are missing.