Genius germ of a possibly fruitful endeavor.
I have had somewhat similar thoughts re a reverse trike set up, as one wheel up front is always less stable where a three wheeler is most likely to experience risk of roll over. That is, when rounding a curve during braking, as the forward weight transfer of inertia then places the effective center of gravity of the vehicle diagonally forward to an area outside of that transcribed by the three wheels base of stability. Acceleration forces would effectively reverse this argument except for the fact that they are never as strong as the decelerative forces generated by hard braking- especially in a lower powered microcar.
Having the 2 wheels up front also gives more lateral space to slide the occupants between, and thus allows a shorter wheelbase and less mass for this concept. I am not clear as to how your design allows for a reduction in length- please re explain.
A seat track linear bearing is certainly doable, as is the double geared steering input to activate the seats motion upon it, provided that some clever linkage is contrived. You would need to mount the cable controlling pedal box to the seat frame. Not sure if it would work better with you hanging off the steering wheel, or would require the more difficult engineering task taking it along for the slide.
Other challenges would include sealing the gap between stationary and sliding body parts for both weather protection and security. (Locking Kevlar gaiters?), and side impact protection. The need for the latter might be partly mitigated by the fact that you would be sliding your self into the inside line of a curve in the road, when you would be most likely to experience an initial impact on the side of the vehicle nearest the outside line of the road curve. This being the case, one could perhaps engineer a shock absorber of sorts connecting the seat assembly to the chassis that would allow for only lightly damped slower steering inputs, while creating a deceleration G force curve along the now fully extended length of the lateral seat assembly track that is designed to best suit survivable to its human occupants. T bone impacts.. not so much.
I have at some time seen/ heard of leaning three wheeled scooters, and even 4 wheeled cars that allow for occupant leaning in curves in both concept and in limited production. Did Al and I miss anything closer to our ideas that has already been tried?