Yes, a simple headscrew with maybe a spacer can lock the sleeve to stay put.

Regarding the backplate. Look inside the piston when the rod is mounted, it has some play to move sideways over the piston pin. That play will also cause a bit of a move of the rod over the crank pin. It is not much with most design pistons but in my opinion it is not what you want to happen.

Indeed you also want to stay the crank shaft in its position, I made a simple long nut to screw on the crankshaft but a simple flywheel nut with a spacer will work also. Anothe trick is making a tube from the bearing up until the flex coupler that will lock up the crankshaft from sliding in the bearing..