OK, that worked fine in both modes via the sandbox. The JS auto-submit removes my concern about incomplete orders in SIM mode. Both modes are now using the normal order completion path, which is considerable improvement, and DPM looks seamless (as it should).
It's probably about time to toss this up on d.o as a dev release and let folks hammer on it. Since this is mostly your own work now, you likely should release it there yourself. If you haven't applied for a CVS account on d.o, you can do that here:
Have you run it through the Coder module yet?
Nice work! This is a very helpful addition to the Ubercart payment options. I expect to use it heavily myself, to avoid dealing with PCI certification hassles.