Dropped Ceiling Configuration and Main Runner Splicing
The advantage of going with 3 main tees is it will get the mains away from the wall farther, using cut 4' tees. this way if the main is above or below the wall angle, the resulting skew the cut 4' tee will have will be no where as bad as the main being 2' closer, using a cut 2' tee.

The main tees simply snap together

Your issue with the 5" shortness, you will need to buy 5 mains total, so you can add the 5" needed. 5" off one end to finish one full run, and 5" off the other end to add to the other end of the next main tee. This one you would turn around so the cuts are in the same side of the room. The second extra piece would be cut for the 3rd main tee that goes in place. You will have a lot of scrap left, but that is the way it goes. I tried recently to use a regular tee to add a few feet to a main, but the connectors are slightly different, so it does not work.

One more tip. You should not necessarily add simply 5" to complete a run. If you want a professional look, calculate the spacing of the slots for the intersecting tees so when assembled in place you will have an even cut tile on opposite side of the room. This means your added cut piece will not be 5", but perhaps 14 1/2" instead. while cutting off 9 1/2" on the other end so the border would be 14 1/2" along those 2 walls. Do the same for the other direction. This is important for setting the screw eyes that hold the support wires, getting them reasonably close over the location of the main tees.

If my math is off, please understand the concept, not relying on what I may have calculated in my head, it hurt a lot.... smoke was coming out my ears....