You get a more even spin if you use circular disks, but it'd be no problem to make a disk with an indentation to accept a rectangular plate, that should take care of most of that issue. But anyway, when I spin square substrates, I only see a bit of nonuniformity right at the corners.

The speeds are controlled to several thousand rpm and can be programmed, for multilayer coating etc.

The coater I have ran $2k or so, but it was brand new. These things come on the market for decent prices whenever semiconductor fab shops retool or whatever.

I typically spin films of a couple hundred nanometers, but it depends on viscosity and spin speed and time. It is also no big deal to drop fluid as the thing is spinning.

I don't have a heating chuck for curing, but the thermal conductivity of glass is very low, so you'd probably just want to blow in some warm air anyway, if you do want to warm the emulsion.