I am a Pentax user for a lot of time and I like to think I know some patterns about how they think and act. Some things I know to be facts, others are just my speculations, Pentax was a very secretive (even too secretive for their own good) company during the Hoya ownership and seem even more so in the Ricoh era. But they (Ricoh) made no secret at all that they want to challenge the "big boys" pretty much the same they did in the copier business. And also they are ready to spend money to make money. Now about the 90 mm I fully agree that is a strange focal, neither here, nor there but beside the fact that Pentax was historically very good at making strange focal length lenses (some say they are just the honest ones in a world were every 47-53 mm design is billed as a "50 mm normal"). Remember the 77 mm Limited (for 135 format) where 77 was "the magical number for a portrait lens" and another big time mythical Pentax lens the 31/1.8 Limited.
So, that being said I have the feeling that Pentax wants to kill two birds with a stone with the 90 mm and make a very advanced design (in terms of elements/groups) which won't be extremely expensive nor big/heavy simply because it is not a fast 120 mm or a 135 mm to start with. I think, as in speculate
that they want to make a lens which is very well corrected for any kind of aberration, with beautiful bokeh and with minimum focusing distance lower than classical 5 elements / 4 groups and 6 elements / 5 groups "old school" designs and versatile enough so it is not a portraiture only item.
About the 6 micron pitch I believe it is the very best for them to stick with it not only for the very important think you mentioned (puts less pressure on the lenses) but also because it is one of the most mature cell processes at Sony (my personal belief from more than a year a go was that Pentax will look for a Japanese supplier and a CMOS sensor for the next MF camera and I stick to that). Since Sony is the supplier for every single other sensor Pentax uses right now but the one in 645D I think it is safe to say they (Pentax) will go the same way. MF cameras as long as they have a prism and a mirror that moves will never be speed demons but a modern CMOS MF camera could make a 135 system obsolete for other kind of work very easy. Regardless of what other MF company chose to do I have a very deep feeling that Pentax will chose a very fast pace of evolution.
You certainly seem to have sources in Pentax! Iím glad to hear the pixel pitch will remain the same; most of the old lenses do quite well with the existing sensor, but might be challenged by smaller pixels. So the sensor will be about 20% larger ~ 50MP? I am happy with the existing electronics, maybe because I was primarily using film before the 645D and had no basis for expectations. Heck, auto focus is a novelty to me!
Looking forward to the wide angle zoom; like Lacunapratum, I donít understand the urgent need for a 90mm.