I liked your idea about putting the AV cabinet under the steps. So I went down stairs to visualize it more and I am not sure if there is enough room by the time the rear seating is there, the riser, and it only being the low half of the stairs. I may remove the wall in the media room like you showed. Maybe just put built in cabinets.
I think I like the entry into the media room being on the horizontal wall. With it on the vertical wall I can see it being difficult to get furniture in.
I laid it out with the bedroom/office on the left side. See the picture. On thing I am concerned about is that it seems like there is limited usable space in the rec room. Because you can't block the doors. But when the bedroom is on the right side those doors are grouped. Then there is a larger area past the media rooms doors that can be used for our bowflex and elliptical.
I am trying to figure out a way to make the wall in front of the air returns removable so I can move that wall closer and still can gain access to them if need be. Maybe some kind of movable bookcase or something.
I also added in where the circuit breaker and cable TV boxes are. So those need to be easily accessible. I have mocked it up in the basement and it seems like a 30" space is pretty comfortable and easy to get to everything.
I envision the access wall as a series of panels (doors) the whole length. Floor to ceiling. Angle iron top and bottom. A few panels could be fixed, but entirely removable, by unbolting them from the angle iron (bolts on backside). That would allow more volume to the room, yet total access to the entire wall length.
Check local codes, but as far as I know the doors into the theater can swing either in or out. In Massachusetts, in most cases the doors need to swing in the path of egress only once you hit 50 occupants inside the room.
One thing I keep debating is removing the closet in the bedroom and just call it a office since that is what I will use it for anyways. The house has 4 bedrooms already, so not sure what the real value is to add a tiny 5th one.
But I do like the idea of having some moving wall/shelving to provide access to the vents. Not so sure to the electrical panel. Only because I would want to make sure I can get to that in a hurry.