Hello again everyone. Had to do some remeasuring (just only for the sprockets and this "gear-rectangle" I guess you could call it, haha.), and tomorrow I should be heading over to my grandparents, to work more on the prototyping of said rectangle. So that should come along more swimmingly, as my grandfather used to be a woodturner/woodworker, and now does woodturning for a hobby/side job.

As for mounting the sprockets to the axles, I turn to what Gary said, with drilling holes into both the sprockets and axle...

7/8" axle is about right. You wouldn't want to go much smaller anyway. As to connecting the two there are a number of options, some people weld sprockets and gears straight onto shafts, you can drill straight through the sprocket and shaft and put a pin or bolt through, there are various clamping systems available or keyways as you mentioned.
Turns out the keyless bushes were way too expensive... But I had read in the Riobotz pdf somewhere that screwing in the sprockets into the axle with screws is a bit of a no no... is that only with the grub screws that bearingshopuk supplies? Or does it apply with all the screws? Because from what I could understand is that it could act as a mechanical fuse of sorts without the need for CNC. A downside is that it could end up breaking inside the shaft and making it a huge pain to get out of the shaft itself?

Any insight would be phenomenal.

DJ.