1. weight advantage doesn't work out. A good aluminum case is very thick, and once you put a few compontents in, you'd be hard-pressed to tell a difference. Looks I will grant, though...alumnium cases just look slick.
2. Few cases have these. Several, like the e4252 and SLK3700-xxx have enough room in the right places that even changing CPUs and such can be done w/ the mobo in place. The few I know of that have trays are aluminum...what's w/ that? Aluminum cases should be worse for that and need more support!
3. Soldering iron and misc. parts can get this done fine. I'l look at somewhere like All Electronics for headers and connectors to make a plug for them, so you can unplug that and be done w/ it, and so it will work later w/ a different mobo.
4. Buy thumbscrews separately. The best are
these, brass, easy to use, tool-less, and don't get stuck hard like aluminum and steel ones.
5. Coolermaster's got you there. Front intakes, as you can tell by the e4252 and SLK3700, don't need to be numbered! There just needs to be space in the front.
6. Don't know here. The Praetorian looks to have a grill. If it doesn't have a filter, it probably wouldn't be hard to add one.
I have yet to see a Lian-li with plenty of intake room, excepting their server towers, which have plenty and filters.