Or am I overthinking it and should use Platewheel Simplexes for all three parts of the mechanism? I ask because I feel like the "dm" bit on the 11T would be a bit too large for a 12mm steel shaft with key to go through and could lead to breaking said sprocket without so much of me breathing on it.
I am not sure I follow. If you go down the key route, you need to account for half of the key height in the bore. I've had keys done in quite thin boss widths in relation to the bore without breaking. I'll see if I can get a photo of the key.

And 2, would it help for me to use a couple of Shaft Collars to keep the shafts from sliding out on use?
I've used them on the old version of Jibril and yeah, two inner shaft collars work. I do advise getting ones where they use two bolts though, single split collars don't work in my experience.