I had a G5RV, and it worked pretty well, It was set up pretty weird to. The Apex of the thing was at about 30' up in a pear tree in the backyard, and one of the ends was about 45' up in the rear part of the yard, and the other end was actually run over the house, and tied down to one of the roof vents. It actually touched the chimney at about 25 ft, and thats why it's down now, it rubbed itself in half on the corner of the chimney (I had saw it coming, but oh well). I used it and had contacts on every continent except Antartica, running about 1KW, but some were 100 watts. it depends on conditions.
When I bought my vertical, I was amazed how well it worked compared to the G5RV. The vertical beat it everywhere except for 40 & 80. The G5RV is now in the garage awaiting repair, and an OCF Dipole has taken its place. The OCF dipole is pretty even with the vertical on 20 and 40, and way better than both on 80.
I'm saying this because there's way better multi-band antennas than a G5RV IMHO, and you should look at the OCF dipole, and the Windom's and the 43' verticals as a project for the future.
1. What effect, if any, is the aluminum siding on the house going to have?
It will affect the tuning and the radiation angle of the antenna adversely.
2. How much of a difference will it really make if I can get the antenna above the roofline? The trees are pretty tall, but I'm worried it's going to be a pain to get the antenna higher.
You will see a big difference if you can get to 40', The higher the better. I believe if you modeled this antenna on EZNEC, you'll see a lower angle of radiation on 20m and maybe four directional lobes. This may be a pain, but I believe it will be worth the effort. Get yourself one of those painters extension poles at the local home center, and buy a 6" roller handle. Bend the perpendicular end where the roller goes parallel to the axis of the handle, with the open end up. This is a great tool for placing wire antennas in trees and I use it to retreive the Dog's frisbee when I throw it over the fence also
3. How important really is it for the dipole to be straight? If you look where the right side is attached, I’d like to attach it the next tree to the south, however I have another large tree in the way. Wife won’t let me cut down any trees.
Based on your diagram, it'll be fine. It wont effect much of anything at all. Route the antenna through the large tree's limbs.
4. What general things can I do to improve performance with minimal investment?
Getting higher in the air will help the pattern, and also keep the ladderline off the ground (very important I've been told)
Cut off any cheap coax, and splice on some good stuff like 9913 or LMR 400.
I wonder why you don't hear much DX. Is it that you have a lot of local noise? Is it the time of day you listen? I can hear DX all the time here in IL, and my antennas really arent anthing special. I would think that even if they cant hear you, you should be able to hear them. Alot of these stations are extremly strong. Maybe you should look through your feedline to see if anythings amiss, using a tuner, you might be matching a bad connection somewhere.
Hope this helps you.