It's quite a pickle, and it doesn't seem to be the popular choice, but I say invite Hortense alone.
She will be happy, your mother will be happy, you will be happy and whatever happens the gossipers will gossip.
And amongst those female gossipers that will attend your party, I'm pretty sure most will be happy not to have Hector creeping them or their daughter/nieces out.
If someone comments on it, maybe Hortense herself will say something like "oh, Hector couldn't come today, but I'm happy to celebrate this engagement!", at least you know she won't say "what an affront, I'll never talk to this family ever again because of this slight" and that's the only thing that matter.
One poster said that she has to live with the fact that she is staying with her husband, true, but I say, cut her some slack, she has to live with her husband! let her have some nice time whitout worrying that her husband will embarrass her and bother every female guest.
If they where actively divorcing but still legally married wouldn't most say that it's OK to only invite her?