not quite the same board, but it never is:
IBM PS/1 2155-593 Restoration
Intel486dx33 also posted a link there, 3rd post, that look like quite the resource, including that pdf manual.
1.) IBM may be notorious for going their own ways but they're not Apple. So chances are many measures and standards are still the AT
2.) see 1, LPX is the key, but the riser slot may correspond with AT or ISA slot 7 (from the bottom in a tower config), putting it about 125 mm from the boards edge. If so, it should work with any desktop case that used to have the riser their, if those do exist. Only pizzabox I have uses slot 6. With anything but the original case, expect to need a dremel at some point.
3.) definitely FPM. I had FPM in my 1997 P1, was still cheaper then
4.) I agree that 16 should be plenty, but trying out couldn't hurt?
6.) I'd assume you'd have to get one of those adapter sockets for any CPU with 3.x volts? Otherwise stuck at dx2-80?
7.) https://octopart.com/hm514260aj-8-hitachi-2257174
Those two beetles left of the cirrus chip are 256k x 16 bits, so 512 kB each.
Video RAM requirements are really straightforward, iirc: Warcraft 2 would have 640 x 480 pixels at 256 colors, that being 2^8, you'd need 8 bit, or one byte, times the pixels, makes 307.200 byte or 300 kByte - so 1 MB should be plenty for DOS gaming and somewhat sufficient for shitty CRT-era Windows.
Have fun!