True, the player can modify the graphics / HUD files and it is tolerated...
I don't think you meet many players in solo... (unless you count system chat)
I don't think external utilities modify game files exactly, although they can interact with the log files to some extent, I use EDDiscovery whenever logged into any of my 3 accounts, it feeds EDSM useful info as well as assisting me...
None of this I think would logically come under the remit of cheats though...
Well you don't think indeed. I'd say, if any file is editable, that means Devs are ok for you to tweak them. I use EDprofiler and it does that.
I could think of voice attack and macros being forbidden in PvP tournaments but completely allowed by FDev.
Having no market connectors means you need to discover market by yourself. Was it the intended gameplay or not, is purely objective at this point.