Hello David,
Interesting project. If it were me I would make the triangular prism column from 25 stacked triangular pieces of say 20 (19)mm ply glued and screwed together for your 500mm prism length. Yes the prism would be heavier -calculated at probably around 9kg Vs 3kg for your fabricated version. When has mass not been a good thing in mounts? (Ok- heavier to carry!) You can add the internal T-nuts you want (good idea) by cutting a say 75-100mm hole for internal access (with hole saw) in the centre of every 4th triangle for around a 80mm vertical hole spacing, or finer spacing as required.
I'm not over-joyed by the timber legs (t-section or not). I'm not sure i'd bother with the t-section legs, just use 190 or 140mm x 45 KDHW (if using timber VS steel) and with say an 8-10mm thick aluminium plate screwed at end to act as a mounting plate (with holes) to the centre column. It would be much easier to make (and be stronger). For added lateral stability you could use a triangular or 3 spoke stretcher type base to join the legs.
As for adhesive, PVA is good for timber of course (esp. furniture), but get the external type by Selleys. You can also use Araldite which is excellent and allows you to form nice large "fillets" where necessary, outside the joint surfaces to effectively increase the bonding area.
Gee... I think I now should make one
Best
JA