The big disadvantage of hindsii is that probably only the tetraploid version is available. At least I have not seen the diploid version yet. But this would inevitably lead to triploid offspring. Only in rare cases can triploids produce seed. I believe procimquat is such a case (but I'm not sure if my memory is correct)
Also other kumquat types are supposed to be early flowering, maybe other types are suitable?
@Marcin I have different hindsii from different sources. If you are interested I will send you some budwood.
@Till backcrossing with kumquat would also have to include selection against such strong dormancy. But in Marcin's case, he is in a more continental climate with warmer summers than our climate. That might be less of a problem for him?
But the disadvantage is indeed that large quantities of seedlings are needed to select for these traits. And that, as always, is a problem.
Backcrossing with Poncirus is disadvantageous in my eyes because Poncirus is already sufficiently winter hardy, but the backcrosses are very unlikely to have improved fruit (it is still not impossible of course) a backcross with Kumquat would first improve fruit quality. With simultaneous strong selection for winter hardiness.
In theority, backcrossing the hardy BC1 hybrids would then have to be done again with kumquat, until winter hardiness and other traits are transferred.
I think this is at least one way, but not the only one.... for time reasons alone, I would go both ways. And it is theory, there may well be other ways.