I made three lengthy trips to the UK before moving here. (6 weeks, 9 weeks and 3 months). I was extremely nervous on my first trip as I was staying with my parents while my divorce and house were being settled and I am self-employed. I was also completely exhausted and a bit upset (as I detest flying). The first time I was questioned for about 10-15 minutes, then told to sit down in the chairs in front of the desks while they called my then-BF. They questioned him for 5 minutes or so, then me again for 5 minutes, and then I was allowed through.
The next two trips I was questioned significantly less, perhaps because I was much less nervous, and by then not only did I 'know the drill' but slipped in a comment that I was fully aware of the visa regs and would be returning home to apply for a fiancee visa. I also stressed that I definitely needed to return to the USA because all the stuff from my old house was currently either in storage or at my parents and I needed to sort through everything and tidy up my affairs before moving.
It sounds like you are prepared. I wouldn't worry.