I think he might have learned to disguise himself, somewhat. I strongly believe he practiced his mugshot stare in the mirror at home, and that he probably also practiced smiling more and looking more human (so as to catch the ladies).
Since he sucked at picking women up in bars (many men do, IMO), he revised his strategy and you may be absolutely right about Mad Greek (even if employees don't recognize him as a customer, he might still have hung out around that area). I suspect he followed more than one group of women, eventually leading him to the relationships among the party houses, where there were no bouncers or managers or anyone to antagonize him with their official scrutiny.
I do think he wanted to kill more than one woman at one time and did not need much to trigger him. I definitely think he was stalking them, but I don't think he was K's mentioned stalker (the story as told by her parents doesn't even sound like a stalker), Did K think she had a new stalker? Did both M and K feel uneasy that night? Since K had been gone from the house for 1.5 weeks, she may not have engaged in a lot of talk about this issue upon her brief return. I assume that prior to her break-up with JD, he too was at the house a lot - providing a measure of "masculine security." BK may have noticed that K was gone and JD not around - time to shape up his own plans.
He would have benefited from having a good friend in his life, but apparently was incapable of that kind of closeness.