Wow 3 1/2 years! That's a project! I'm sure you got the award for longest thread from the Mods...
That partition is typical construction, done to keep the weight down. Using wood glue and/or PL construction adhesive adds considerable strength with no weight penalty. FYI on window installation...ChemLink is a good choice for sealant but not so good if you ever need to replace that window. It's also easier if you have the interior paneling installed, the inside trim ring is designed to be the clamping pressure on the window so you don't need actual clamps.
Looks like the project is coming along nicely though!
