Greetings CTM,
You are correct, the very strong and very weak signal situation is a difficult one. This type of situation is one of experimentation. Luckily, all your signals with the exceptions of WCYB and WBRA are all UHF.
First the preamp. With 73 db signals nearby and being 20 degrees off, I wouldn't run one. Remove it and see how your antenna does strictly using its own gain.
There are methods to null, or modify an antennas ability to receive so that the very strong signals are attenuated or lessened in a particular direction. Here is some information on this technique. It requires two antennas and spacing them:
http://www.hdtvprimer.com/ANTENNAS/ganging.html#TAT
There are some other options such as filters that block or greatly reduce signals on one specific channel. Those aren't cheap. Rabbit73 can provide more information on this.
Further complicating your situation is that your weak signals are very weak. Generally around 7db signal strength is when atmospheric conditions can start to affect the signal so that it comes and goes. Could you run a TV fool map at 50', 75' and 100'? If you can get the very weak UHF signals to around 10db signal strength, your chances will improve greatly.
Your situation isn't going to be one of "plop a 40 dollar antenna in the air." It's going to require some testing, engineering and creativity. It's doable, but may be a bit more intensive than you are looking for. And yes, trees are a signal killer. It's strongly advised you get above them or trim them so the antenna has a clear view of the horizon in the case of the very weak signals.
Cheers.