UA's safety and security policy requires that passengers remain in their ticketed cabin absent crew member permission. Once that policy is filed with the FAA, the failure to follow it is a violation of FAA rules. This is up to the carrier. By way of example, it is not an AA safety & security policy (not to suggest that it is OK to cram 3 passengers into 2 seats).
It is also a violation to seat a person over age 2 (in chronological, not behavioral years), so yet another violation. This one matters in case of unexpected turbulence. While one could say that it is the idiot's problem, it is also your problem if the idiot goes flying and lands on you.
The FA ought to have given the trespasser the boot the second he showed up without permission. The fact that it was repeated and that you had to report it shows that the FA was not paying attention to something that UA considers a safety and security issue (whether it is, is irrelevant and not the subject of this thread).
As a general matter, I would have said something to the FA about 2 minutes into episode one and made it clear that you are going to overlook the failure if it is dealt with immediately.
Despite the fact that the flight is now over, I would send UA a short note making it clear that you are not scrounging like a beggar for "compensation" but you do want UA to speak with the FA responsible for that cabin.