The steel-collar method to clamp the bottom-bracket shell around the eccentric will work only if the shell has a slot cut into it, which doesn't seem to be the case here. What I see is a billet of aluminum welded to a solid shell, into which set screws have been threaded. In this design the ends of the set screws pass through holes in the shell to bear against the eccentric and when tightened prevent it from rotating. What appears to have happened to the OP's bike is that the weld has failed under reaction stress from the set screws as FBinNY says. (As the screws are tightened, they want to lift the billet off the shell, and only the weld prevents that.) Our old Santana uses a similar design except that the the shell is steel and all one piece, just formed with a thicker "rib" for the set-screw threads, no welds to fail.

I agree with FBinNY's suggestion to cut a gap in the shell and weld two pairs of threaded "ears" each side of the gap. Two bolts that bridge the gap will compress the shell around the eccentric. Many tandems use this design and it works well. My only quibble (to avoid confusing the OP) is that using a steel collar to accomplish the same thing will not work until the gap is cut. Really, all depends on whether the crack we see in the photo is just the weld failing between the billet and the shell, or whether it is a crack through the shell itself. I think it is the former. The frame could be salvaged (if not replaceable under warranty) if it could be welded successfully as FBinNY recommends.

Agree also that Bushnell eccentrics are excellent but unlikely to be cost-sensible on an entry-level tandem even if one would happen to fit the OP's frame. The ears and bolts compressing a split shell are the way to go.

I'm hoping FBinNY will give the last word on this since s/he has more experience in fixing this sort of thing. My input is merely as a tandem driver.