Lesschumras
I don’t think it matters how many dates a couple goes on in terms of dating two at once interfering.
IMO, all it takes is one date to begin comparing. Again, it’s my opinion, but I speak from personal experience, not my general Hashkafah.
The second example you cite is completely different. The former date is no longer available. The girl is equally connected to both (meaning she can embrace a life with the second as easily as she would with the first). That would require her to be able to let go of the past, a phantom guy, and move on.
In my story, there was no phantom guy. They were both very much still there.
Perhaps there is some measure of maturity involved in being able to prioritize more practical marriage traits over others. But, when put to the test (and it’s a much harder test when two are in front of you) I think many would be swayed by looks unless c’v they’re blind.