Awesome. I think the 10mm would be good. I don't think I would go too much lower. It's not just the merging but I think also magnifying any ill effects from the prisms. A Barlow before raises the focal ratio. The OCS already brings me to f/16 and there is no chromatic aberration being monochromatic, so I have not worried.
Sorry for the topic drift. But the binoviewer makes a bigger difference in H-alpha (and solar white light) than in anything else I have done in astronomy. Not as good as single stack to double stack, but feels like halfway there as I think Marty or BD has mentioned before (sorry if I misattributed).