So...Im not a bike person,so cant compare and relate my ideas to bikes,but I have built several cars over many years..
Can I bend your ears please??
Im thinking that I need a axle around 25mm diam.of EN16 steel? or possibly machine down a car half shaft?
Im going to build a hub in aly(the correct grade) or steel, to accomodate a brake disc sitting inside the back of the wheel ala the usual car configuration.
The hub will have a ball race bearing at each end.

If I fit the (chain drive) sprocket to the same side as the brake disc,outboard of the tyre of course ,this will make the process of wheel removal slightly easier,although will still have to strip down the axle to get it out of the swing arm.This will also mean less complicated machining due to not having to sandwich the wheel between the hub,and also I wont have to worry about finding,or having machined a wheel rim with 2 flat surfaces,front/rear.Im worried about the closeness of the sprocket to the disc because of oil contamination from chain to disc.Im thinking about fitting a aly or plastic disc(as per some go karts) between the two. OR should I just mount the sprocket on the other side of the wheel?
I would like to use a single sided arm,but due to chassis constraints etc Im going to build a 'conventional' arm from heavy wall 2 x 1 box section,with widely spaced pivot points and using heavy duty rose joints.I may brace the arm further and add an extra pivot point each side.
Im intending to use a Mondeo disc/rear caliper,as they (apparently) have the h/brake mechanism built in. I will use a hub pcd (probably Ford)that will allow me a wide range of wheel diams and widths to experiment with if necessary.
One last thing! Im using a chain from a transfer box to the wheel. It will drive the wheel on a 1:1 ratio.Approximately what size sprockets should I use? I m thinking about 9" diam?.

Opinions GREATLY appreciated please!
Thanks..........

Why not use thick wall tube for the axle?

There is no problem with putting the brake and socket on the same side but why? You have to take the lot apart anyway to change anything. I would put them one a side and use spacers to keep it all central.

There is no need to use rose joints on the rear swing arm, use a single cross tube with 'top hat' bearings then it can be mounted on two chassis mounted tabs with a long bolt.

Rather then 2 x 2 use 2 x 1. It will be strong enough and lighter.

The size of the transfer chain sprockets depends on the size of sprocket the transfer box can take.

Post some drawings to show us what you are doing.