No short answer...
Re (1), which I would re-phrase as "the LH occupant centerline to the RH occupant centerline", the design is somewhat inconsistent. I think the folks at Van's are even a bit confused about it. I've spoken with them about it some time ago, but they weren't very interested in getting to the bottom of it. Studying the plans and measuring the prefabricated parts, you get a different answer depending on what you consider to be the reference.
The seat backs on the plans are shown to be positioned 19.5" center to center. As I recall, this is not explicitly called out anywhere, but can be derived from other call outs and measurements on the scale drawings. BTW, the builder can also alter this dimension somewhat by deviating from the plans when installing the hinges, and I know some RV builders have done this for their own reasons. For the sake of this discussion I'll assume you're locating the seat backs per-plans (as am I).
The seat back supports on the upper member of the bulkhead are positioned 20.0" center to center by pre-punched holes. I'm guessing that this was a design error on the part of Van's, since it also causes an interference with the roll bar supports in the tip-ups that has to be corrected by removing some material. If it were 19.5" center to center, then the supports would match the seat backs, and there would be no interference with the roll bar.
The prefabricated rudder pedal weldments have approximately 22.5" center to center spacing.
So between the seat backs (19.5" or 20.0") and the rudder pedals (22.5"), the occupants' bodies are made to face slightly outboard instead of straight ahead. I'm not at a point yet where I have all this installed, so I don't know if that will be noticeable or not when sitting in the airplane. But still, that creates some ambiguity as to where the ideal occupant centerlines should be projected onto the instrument panel.
On my panel, I currently plan to use 19.5" as the nominal center-to-center spacing, i.e. consider each occupant's centerline to be 9.75" outboard of the fuselage centerline. Partly I chose to use the smaller of these dimensions for this purpose because it will keep the instruments on both sides more visible and accessible from the opposite seat. But I haven't done a mock-up yet, so I can't really say yet if that's the best answer.
Re (2) and (3), the seat backs and bottoms are angled, as is the instrument panel, so your question about distances between them is somewhat ambiguous. The seat backs also have several possible positions (9 possible combinations of the 3 hinges at the bottom and 3 support depths at the top). Also, your choice of upholstery will make a big difference. So I can't really help you with that.
I hope the info I gave you on (1) proves helpful. Good luck! But I'm curious, why are you planning your panel now if you don't even have your cockpit area built yet? I would wait to plan the instrument panel as late as possible in the build process, so that you can take advantage of what's available then in the fast-evolving avionics market.
-Roee
