Welcome to the forum, by the way.
This entire thread describes how troweling is far from ideal. You can certainly use whatever trowel you like, however.
I mentioned the 3/8" bead, which I believe is standard in all the installation...
I don't recall saying "because I said so." This is part of the problem buying technical items online with bad information / no support.
The 50 degree temperature would be better moved to 70 to aid in compression.
You want 3/8" thick...
The IB-3 brackets are spaced every 4-5 feet to connect the top plate to the joists. When framing a new wall perpendicular to the joists, you may need to install blocking between the joists to provide attachment points.
The walls you...
Looking at the specs, the fixture is 7 1/8" tall. If using clips and channels, that will essentially increase the cavity depth by 2 1/2", assuming double 5/8" drywall. So the fixture gets dropped by that amount. So now you're at ~11 1/2"...