I do have two structural connections from the floor to the roof, right behind the driver's seat, as well. See the side view in this drawing:
With no side doors (ideally, if this entry scheme works), then the only structural "holes" in the front and sides of the car, are the windows. With conventional side doors, the openings are much taller, and the doors are essentially disconnected structurally; and they do not support the roof much. The windows pillars are spanning a much shorter distance, and so they do not have to be as thick as a "normal" car.
The top hatch door and the rear doors would have multiple latches to structurally join them to the rest of the car. So, I am aiming for a composite foam sandwich structure (to provide good strength, light weight, and thermal insulation), with continuous wraparound structural cage on the front and sides; room for 4-5 people (the front 3 figures are 6'-4" tall, and the next 2 are 5'-8 and 5'-0" respectively), enough room in the floor for a ~32-50kWh prismatic battery pack, hopefully weighing 1600-1800 pounds, and having a CdA >3.75 sq ft and maybe as low as 2.75.
The rear hatch would raised and the rear door(s) would swing open, and you then step up on the rear floor (about 14" up IIRC?), and after you move into the back, you turn around and close the rear door(s), and then walk down the "aisle" past the staggered offset seats, ducking a little more as you get closer to the driver's seat. You then sit down from the left side, sliding your butt into the seat (the space between the driver's seat and the one passenger's seat on the left side of the car is bigger than the right side). You then push a button/pull a lever to drop and close the rear hatch door; and it latches in at least 3 places. So, it is firmly "joined" to the sides and rear of the car by the latches, and to the top by the hinges. Please note, the hatch door provides some sheltering from precipitation, so the inside of the car is not directly getting rained/snowed/sleeted in...
Oh, if a second means of egress is required, I'd put a hatch door in the roof just above the driver. And if the rear hatch entry proves impractical, then I'd go with side doors that have multiple latches. This would add some weight, though; and I don't think it would be as strong, either.
The other purpose/result of the staggered/offset seats is the legroom is flexible, and people can stretch out to the side, to help avoid getting cramped or uncomfortable. Also, note these seats are curved mesh surfaces, stretched over frames; to provide comfortable ergonomic seating (the proper angles and shapes are key!), while taking up a minimum of space and good air circulation.
Something I have forgotten to mention before now is the frontal area is shaped somewhat close to square -- this encloses the most interior volume for a given frontal area, and that makes it easier/more practical to use. You can get upright seating (which I find very comfortable), and that in turn allows you to fit more "rows" of seats than you otherwise would. This car is less than 14' long, and just 5'-4" wide. Naturally, I'd use side video mirrors; or optical mirrors on the inside, to avoid the additional aero drag.
I do hope to build it, sooner rather than later; but if you want me to send you the SketchUp model and/or the CAD drawings, so you or someone you know wants to build it, then I'd be happy to send them to you.