As long as the axle breaks inbd of the bracket (which they usually do) then the disc should keep the axle from sliding all the way out. If you break an axle on a road course in a corner, it might be a different story. Like I said, it’s extra insurance if hits the fan.
I’m Definitely a fan of stuffing the highest spline count you can inside the housing.
Depends on the speed and the size of the tire. I've seen plenty of axle failures that just ripped the brake right off. I'm passed the point in my life where I willingly rely on a 25 year old rusty brake caliper bracket to save my life to save a little bit of money. For me, 33 splines was the limit. There was a massive difference in price between the Eaton Tru-trac 33 spline differential, and the Strange S-trac 35 spline differential. Of course the Strange differential only worked with Strange axles which only worked with Strange reluctor rings, which only worked with Strange bearing retainers, which only worked with single piston calipers. So the extra 2 splines would have severely limited my brake options going forward, which would have likely causes more hazard than the slightly smaller axle.