What were you trying to use, if you don’t mind me asking?
For problem 1, have you tried using a Chrome Incognito session?
For Option 2 I just tested this, and my very mild swear word did appear in the livery screen, though the image of the plane was blank for some reason.
When I spawned this is what I saw. It really didn’t like messing with this, and I think I may have changed one too many fields. 
But my custom tail number came through anyway.
I shouldn’t have touched the UI, or texture elements. 
So yes, it turns out you can put something that might be considered offensive, but only you will see it or hear it phonetically. You must also turn off the custom tailnumber, as that overrides what is in the “aicraft.cfg”.