I will try to answer some of your questions.
First, it sounds like you are needing a screen that has a modest gain while still providing you with a fairly wide viewing angle. Having said that you will probably want to look at a screen surface with a gain of about 1.3-1.5. This will give you a little boost but not too much as to accentuate a hotspot and reduce your viewing angle by a great deal.
Speaking of viewing angles, most of the screen manufacturers measure gain in what they call "half-angle" or "viewing angle" the terms are used synomously. What it means is at what angle from the center do you have half of the brightness you started with. Therefore, if a 1.3 gain screen is stated to have a 45 degree viewing angle. At 45 degrees from the center it has a gain of .65 . Hence you would have a viewing "cone" of 90 degrees since you have to include both sides of the screen center.
As for bringing a manual 4:3 screen part way down, you should not experience any wrinkling by doing that.
To answer your last question, yes indeed there are electric screens that will do what you are asking. I believe that Da-Lite and Stewart are the only two that make electric "roll-up" screens that will mask out the top and bottom of your screen. Based on what I have seen, I personally like Da-Lite's version. The reason it is preferred is because their masking is done automatically. The Stewart has to be done manually through relay timing. Da-Lite has a cool system that is preset and goes to the correct aspect based on which one you choose. Speaking of which, it does four different aspect ratios (4:3, 16:9, 1.85:1 and 2.35:1). The control for this screen is a wall switch that is low voltage and can be interfaced very easily with your favorite controller. If you need to know more about it, check out their web site at www.da-lite.com
. The screen you are looking for is called the HORIZON.
Hope this helps,