The 26" is a decent enough aisle space, but the bad news is you have only three seats with very good audio potential. And there's a bunch of seats on room boundaries,
and right on top of surround speakers. And even a pair buried in corners.
Not sure if you could switch directions at this stage, or even want to, but if the back room is simply a home office, I would have taken a bit more room length
and use a stage to hide a toilet relocation, without breaking the floor.
The real cost is the shrinking of the room behind the theater, and the amount of riser to build.
I myself like this media storage, but I'd do stained hardwood. It might be of interest to you for that back closet. It could be something substantial, fully custom
fitted to the space, and not expensive to accomplish.
A single intake and a single vent, might not be up to the heat load, either.