(1) you can get black stems/headsets in 1" threaded. it's just not the default color like it is for threadless.

(2) i don't think the availability of parts will be an issue for quite a while given the interest in older bikes in general. whenever i need a new threaded fork, i just go the lbs and they look around in the back and always come up with something. all seven of the bikes i have (2 old commuters, entry-level track, custom track, cross, mtb, road) all have threaded forks. and 4 of those are only a few years old.

(3) i wouldn't ride a carbon fork on the street, even if i could, but that's just me. one less thing to worry about.

i guess you should go 1-1/8, but i don't think the disadvantages of 1" are really that bad.