[Flashbang]

Hi
Progress is on going then

Your timber framing is very strong, is it 2" x 2" timber of larger?
I use 1" x 2" PSE narrow edge up to the underside of the baseboard top. All joints are very simple square cut butt joints with PVA woodworking glue applied before assembling the two section together with the aid of two suitable length 4mm wood screws into each joint to hold them.
Legs used to be 2" x 2" but now I tend to use CLS timber, sold really for stud wall construction but is ideal of layout legs, in a metric size of 38 x 68mm.
I have made my baseboard timbers on grid of no more than 15" square to ensure the 9mm ply top wont sag over time.
Once the ply has been glued and pinned down the whole structure becomes very rigid.

 

[Flashbang]

Hi again
If you really want to smooth off those walls, a very quick way is to use Plasterboard as stated. You don't even have to plaster skim it as its in the loft!
I would use 9.5mm thick plasterboard and hold it in place with large dobs of "Dry Wall Adhesive". You mix the adhesive to a very thick consistency and then apply some thirty or so (for an 8 x 4 sheet) thickish mounds onto the wall in the area of the sheet boarding and then press the plasterboard into these dobs. Suction holds the board in place as the adhesive dob dries. This is how most homes are constructed where a plasterboard finish is required on a rough under surface.
This site gives the method of working etc How to fix Plasterboard