You only need 2 couplers for the Z axis threaded rods. They will attach on the each zstepper motor shaft and then will be coupled to the threaded rod. Just like in OME's picture.

I would stay away from the wood glue, the kit is actually very rigid, much more than I thought it would be. The main reason I would stay away from the glue is to allow you to easily take something apart if you need to access, adjust, or replace any parts.

I definitely could be wrong but the aluminum on the heat bed might make it take very long to heat up. I guess this could hold in the heat longer once it's heated up but essentially, it sounds like an aluminum plate above the heat bed and below the glass would act as a heatsink during the initial heat.