Re: hub breaks?
Step - 1- take the arm off before attempting to mount the sprocket.
Then what those guys said about the arm upon reassembly.
I have a similar setup- coaster brake, cruiser bike. When you put the sprocket on, you will swear that there is no way that the sprocket could be tightened enough to actually seat around the hub- but if you do it right, it will and I think that is a must. That way you know the sprocket is centered on the wheel.
Also, center the bolts between the spokes, not rotated so a bolt touches a spoke. If you tighten the sprocket correctly, the rubber spaces will grip the spokes and the bolt will never touch the spokes- if the bolt rests against the spoke, there is undue stress on the spoke at that point.
Remember- from the outside- it's sprocket- rubber spacer- spokes- rubber spacer- metal plates nuts (lock washers, loctitie, etc. of course). I don't mean to be basic- but I have seen every combination of the components- even some wrong directions on vendor websites. I would suggest replacing the original sprocket bolts with good quality bolts of the same size from the hardware store- the originals stretch and break too easily.
It is not easy to get the sprocket centered so it will surround the hub, but tighten the bolts a little at a time and stagger the sequence. What I do is take a Sharpie marker and number the bolts 1-9. I then tighten in this pattern, a little at a time: 1-5, 2-6, 3-7, 4-8, 5-9, 1-6, 2-7....lather, rinse, repeat. Make sure that as you tighten, the sprocket is staying centered and not moving off to one side. If it does, loosen, center it, the keep at it. I top out at 12-13 ft. lbs, but other will say 15- whatever works.
You will have to deal with the dust cover for the coaster brake in some way. Some say grind the sprocket hole to clear it- if you have the machine tools to make sure the hole remains centered, fine. I say, grnd the edge off the dust cover so it just barely clears the hole in the sprocket- it will still seat over the hub assembly and keep the schmutz out and because the sprocket hole was not ground off center- it still will accurately remain in the exactcenter of the sprocket and the sprocket will have nowhere to go, but stay centered on the wheel.