On #2.
The foam is the same stuff, but the cut of the seat base is different. Not curved on the inner front corner to fit tighter to the "hump", standard bucket foam leaves a gap. I franken-foamed a et of ACI and some off cuts from OEM and then put a thin layer of 1/2" medium density foam over the top to smooth any seams. Came out fine. Not sure what you would use if you were trying to fabricate your own, I'd go repro and modify to suit, go to a local shop and pick a foam of similar density and cut-to-fit.
On #5
I ordered some '68 Ivy Gold comfort weave some time ago a year-ish?. They sent me samples in Oz, I picked what I wanted. They shipped it direct to TMI who made me a custom set of covers. Very happy with the results. And yes, they noted that black is becoming very hard to find, and may take some time to source.