Well if you are going to cut off the domes and put on flat end plates, then you can increase the thickness of those plates which will help with the stress issues you will encunter with the original tank wall thickness. I disagree that a continous axle would be a waste. With a stub axle ALL the bending stress would be borne by the tank skin and the point where the stub axle is attached. With a solid axle, a good portion of the bending stresses encountered with starting and stopping the tank would be borne by the axle itself. Also a 10" diameter cup out of 1/2" steel, while a good idea to re-enforce that portion of the tank skin, would be a pretty hard thing to fabricate, at least for me it would. It would require tools and time that I would guess most of us don't have readily available. With changing to flat sidewalls, it is easy to add a doubler plate around the axle to re-enforce it. You should be carefull with a partially filled tank, particularly when stopping. You will stop the roller, but the water inside will want to continue to move(free flow effect) and can lead to some unpredictible movements as the water sloshes to a stop.
I personally don't think that a roller is going to do all that much for your gopher issues. As mentioned, I think the resulting soil compaction will actually do more harm to your grass by restricting water being absorbed and air/gas exchange at the roots. I think you might find a dethatching rake pulled behind your CUT more effictive at collapsing tunnel runs and re-distributing the soil from the mounds.