A BIG +1 to what Dennis Foley has to say.
Another reason you want to be off the surround speakers, is how you set the volume of a surround speaker, close to a seat?
Set it low so the nearby seating doesn't have that channel jump out of the surround field, and is it possible for the furthest
seat to hear it? Raising the surround helps a bit.
All those extra bodies can be expensive, in terms of cooling too. So you need to beef up the cooling. (450 watts +, per body, per hour).
And those extra bodies likely mean more air movement needed, at the vents, which mean noise. And you really don't want that.
I'd want to trade off those occasional seats and move that $$$ over to the projector. I'd be thinking a bigger (and properly centered) DIY
AT XD screen,with a 3' deep AT space. Build yourself some SEOS speakers, and subs, and give them a simple flat black finish. Flat pack
cabinets and prebuilt crossover options make this a pretty simple DIY option. And you have dynamic speakers, that will hold up to the throw
distance for all the seating and be able to play at reference levels. (Another potential diminishing return for that third row occasional seating.)
If and when you need extra seating, why not just bring in a few chairs? A friend used inexpensive IKEA Poang seats, in this manner.
Inexpensive, comfortable and light weight.
I'd also want to put a single door at the entry, and make sure both doors have some mass and gaskets. A low noise floor in the room, is important.