1) The question is not whether you are over-reacting, but what the plans are for the future. It would be reasonable to explain to your wife that you don't think this arrangement can work out much longer. School should be out or nearly out where you are by now, so this might be a good time for the neighbor to relocate, especially if her house won't be ready any time soon. You needn't mention those twinges you feel when the neighbor is bouncing around in her tank top with no one else home.
2) No brainer. Sit the assistant down, and state that your weekend activities are off-limits to his inquiries, unless you bring it up first. Remind him that he has access to your calendar for professional reasons, and that your attendance at "Dumbo the Musical" has nothing to do with his duties.
3) This one I can relate to, oh so well. I've been both mother and daughter in this story. There's no need to mention marriage to your parents until it's at least time to make some plans or buy jewelry. However, they may well feel a lot better about him once his is officially a permanent part of your life, and not just a live-in fixture. In the meantime, remind your parents that many people with 2-year technical degrees make far more than many people with advanced degrees. However, even if his earning potential never approaches yours, you will not tolerate their attitude. They don't have to like him, but they must be polite. If they refuse to do so, they have made their choice to distance themselves from you.
4) How very understanding of you. He remembered your birthday; he doesn't have much money; you want your present, darn it! I'd handle it lightly. Something like, "Hey, I just remembered! You were going to give me that elephant lamp. Did something come up?" Or, you could just get over it!