Large format porcelain tile on 32" centered joists?
Sorry, I have been away with a hip problem and this Question is a tough one, but it does have solutions. Independent companies have their own strict guidelines and straying from them can alter their warranties. 32" on-center floor joists is huge and there are no warranties that I am aware of that can break/stretch the rules more than a bit with this one. On the plus side, I would ask if the joists were parallel or perpendicular to the adjacent hallway walls. In either case, possible flexing of the plywood between the joists could be worrisome, and eventual hairline cracks would be predictable. Hardi-Boards are installed with non-modified mortars and tiles are installed onto the boards with Quality Brand Mortars suitable for bonding ceramic to HardiBacker. Skim Coating AKA "BackButter" is essential on every tile.
I am assuming that 11/4" tongue and groove is a typo and that it most likely is 3/4" T&G. I would highly recommend using a 1/4" X 3/8" X 1/4" rectangular notched trowel to spread the tile setting mortar. Straight no-tooth straight edge of trowel should be used to BackButter the tiles and then a "light" combing of mortar using a 5/32" X 3/16" V-Notch trowel to assure you that all errant air is displaced when the tiles are laid, pressed in and "beaten" with a TAVY Tile Mallet.. The tile layout, parallel or perpendicular to the side walls, is also a concern. And it would help me if you could tell me which way the floor joists are running. Please get back to me here.