Actually, that's almost a cube given the 5.5 to 2.24 part of the ratio.
> Line all walls with 4” rock wool than plaster walls – good idea? <
If you put plaster wallboard over the rock wool you'll negate much of the benefit.
> (B) Corner bass traps – most of the plaster wall will be replaced with 5/8” plywood (to allow it to resonate). <
Resonating is never good. In your case the best choice is your own:
> One option is just fill with rock wool <
> (C) Some kind of diffuser/deflector/absorber between the speakers on the front wall – actually I’ve no idea what should go there, but I’m pretty sure something should – any ideas? <
The front wall is the third most important place to treat for reflections. First is the side wall and ceiling reflecting points, then the wall behind you, and finally the front if still necessary after the first two are done. If you think about it, treating the wall behind you solves most of this because little sound will then get back to the front wall.
> (F) Panel bass traps as per Ethan’s website <
Those panels are more appropriate for larger rooms. They work very well! But fiberglass-only traps are great in a room like yours and they cost less and to make if your plan is to DIY.
> (G) The whole back wall is one massive plaster wall/rockwool construction about 1.5ft deep with lots of niches for the CD and record collection. I believe it will give a combination of diffusion and absorption <
Not likely. With a wall so close behind you absorption there makes a lot of sense.
> (H) Floor: one option is wall to wall carpet, other is wood, in both cases with a thin layer of rubber between it and the current original floor tiles. Which would be better? <
Probably carpet.
--Ethan