I'll answer your questions with what I did ... that doesn't mean there aren't valid alternatives.
Q. With all speakers having front ports, what distance is needed between the screen (woven) and the front of the cabinets?
A. The baffle wall / screen distance is more a function of high freq than bass ports. Recommended minimum is 150mm or thereabouts.
Q. Assuming the screen is mounting directly to the baffle wall, the speakers themselves will need to be recessed further back from the baffle - does this not defeat the purpose of the baffle if there will be a "step"?
A. Don't do this ! The speaker baffles must be in the same plane as the baffle wall. You'll need to devise a screen mounting system that brings it forward away from the baffle wall.
Q. Do the front of the cabinets need acoustic treatment to stop back waves / reflections from the screen?
A. My baffle wall is covered with 25mm black polyester insulation. If the speaker cabinets are large then it would seem sensible to cover them as well.
Q. Should I build a cavity/recess slightly smaller than the screen size, and then fill it in around the speakers to take up the space (similar to a Krix MX system)? - or, build the cavity to perfectly fit the speakers tightly?
A. The holes in the baffle wall for the speakers should be a tight fit within practical limits.
Q. Does the back of the cavity/recess (behind the speakers) need to be closed? I ask as I need the ability to get in behind the false wall/baffle (~700mm deep) in order to potentially service windows/blinds in the future.
A. My baffle wall is wall to wall and floor to ceiling. Access behind is by climbing through a speaker hole. I haven't been in there since the room was finished ~ 6 years ago.
Q. Does the space/cavity behind the baffle wall need to be treated or filled (insulation etc)? Assuming the back side of the false stud wall needs to be covered (plasterboard) or is there any reason this can't just be left open? (It will be hard to get in there and hang plaster afterwards).
A. Mine is empty. The back of the baffle wall framing doesn't need to be covered.
Q. In many ways I am asking if the rear cavity of the baffle wall needs to be completely isolated from the front side?
A. Mine is.
Q. Is there any reason / benefit to using multiple layers of plasterboard/green glue in this application (I suspect not)?
A. My baffle wall is lined with 2 layers of 18mm MDF.
Q. From a 2-channel perspective, what effect would recessing the DD67000 into the baffle on imaging and soundstage? Is this a 100% no-no for 2-channel? The plan was these would be placed out further from the baffle wall in typical "toed" positions, but I've had the thought of creating recesses behind them to partly allow them to sit back further. Worried what this might do to 2-channel use though, along with bass issues caused by having recesses.
A. As above - don't recess the speakers into the baffle wall. If you want toe in then crank the baffle wall - this is how mine was done.
Hope this helps.