1. A gray/black screen with some gain seems like a good fit for your situation. If you are going to ceiling mount which sounds like a good idea for your situation you'll need an angular refletive screen. I haven't used a Black Diamond but spec wise it seems a little better than a Stewart Firehawk which I have used and either could work for your situation. Stewart's new 5D might be an option as well, but I haven't used it yet. I'm going to Stewart in Jan. for their training so maybe I'll have a better opinion then.
2. A good screen makes a difference especially in this situation. I wouldn't skimp here. In a dark room with dark walls and ceiling an inexpensive white screen would satisfy most but in your situation a good screen is worth the extra $.
3. The max a BD screen is seemless is 113", I'd try to stay seemless if possible. I'd look at something around that size to have good picture quality with a good amount of light. Also, keep in mind that bulbs age pretty quickly and will often have less than 50% output by the end of their rated life. Everybody wants the biggest size screen they can fit but sometimes it is not always practical. And what about audio? Being and audiophile this will be important to you. Getting the biggest screen size so you can put speakers into corners, and the center near the floor or ceiling is not going to give good audio. Just another consideration, bigger is not always better. A big acoustically transparent screen with the speakers behind the screen for good audio and video, now your talking. But bring money because you'd need a PJ with lot more lumens:-)
4. The further back the projector is mounted the less lumens you get out of it. You lose about 25% of the lumens from the closest position to the furthest position with the JVC projectors. But mounting further back can often reduce hot spotting with gain screens, depends on the screen. There are tradeoffs with mounting positions but something to keep in mind when determining which screen and size. Use one of the online calculators for figuring foot lamberts and try to get at least 30 ft/l with a new bulb for your environment. With ambient light more ft/l is better. The JVC has enough lens shift that ceiling mounting shouldn't be a problem and you wouldn't need keystoning.
Hope this helps.