I'm curious if/how public events get more difficult as additional, or higher level, guardians link/join the event.
I don't see any real source for this, so this is just my observation. Public events don't seem to directly get harder as more guardians join, however for events which don't have a set number of waves (defend the warsat, prevent sacrifices, etc, including "enemy is moving against each other" events), each wave gets progressively harder. In Skywatch especially, if there are a few level 30's they often clear waves fast enough that the last few waves will just be Hallowed Knights, Hallowed Wizards, Reaver Captains and elite Servitors.
It was also pointed out by MrDobilina that for fixed length events (prevent Vex sacrifices, for example), the wave difficulty seems to be based on the time elapsed rather than the number of wave clears, in our experience Axis Minotaurs will start spawning eventually even if you are alone and it always ends with some red and yellow bar hydras.
Virusboy: Patrol Missions are only the level in which your actual missions take place the first time around. So the wave starts at that level then progresses every level till 3 levels above the area level.