Hmmm... well, I could also see another solution:
We agree to Quebec becoming an sovereign nation recognized by the UN, with the ROC either keeping the name Canada, or calling itself English Canada, or some other name. Quebec and the Rest of Canada maintain a common citizenship and passport.
Now for the rest of Canada:
Of course within our current constitutional framework (i.e. in accordance with the rules necessary to make any changes to our constitution):
1. We adopt a new Constitution fully in conformity with the Universal Declaration of Human Rights.
2. English becomes the sole official language of federal government administration. As per point 1 above though, this would limit itself explicitly to federal government administration, with the federal government having no jurisdiction over commercial signs for example.
3. Again as per point 1 above, no preference could be given on the basis of religion, so the separate school system goes. As a token of good faith, we could always adopt a voucher programme for example, though again this is not something I'd support in the constitution but merely in law. We could ensure though that nothing in the constitution would ban such a possibility so as to put Catholics at ease over this concern.
4. In collaboration with all other Commonwealth realms, we try to introduce an elective constitutional monarchy. Failing that, we keep the monarchy we currently have.
As for the new sovereign state of Quebec, obviously its constitution would be its business for the most part, except for the fact that its constitution too would have to recognize Canadian common citizenship and a common passport, revocable only though mutual consent with Canada (and our Constitution would obviously have to include the same thing towards Quebec). Beyond that though, Quebec could adopt whatever it wants into its constitution.
I think such an arrangement would wash English-Canada's hands of any heavy handed tactic on Quebec's part, leading to any international criticism of Quebec language laws for example becoming a matter that would embarrass Quebec only and not English Canada. Meanwhile, the guarantee of a common citizenship would help to protect Canadian access to the St. Laurence River at least to an extent, and Quebec access to a larger North American market.
Any further agreement between English Canada and Quebec would be strictly on a legal and not constitutional level. Constitutionally, each country could have its own military, regulate its own resources, and have its own currency should it wish to do so, any shared policy on that front being purely mutual and voluntary, only on a legal and not constitutional level on either side as long as it is mutually beneficial, and therefore cancellable at any time.
While Quebec wants sovereignty, I think it also recognizes the importants of some kind of guaranteed access to a larger North American market. And while English Canada may feel weighed down by Quebec, it also recognizes the importance of guaranteed access to the St. Laurence. I think this is something both sides might go for.
Clearly? Not so clear on this end. Could you explain why our constitutional matters require the consent of foreign states?
The only way for Canada to change the rules of accession without breaking from the monarchy altogether would be through unanimous consent of all Commonwealth Realms. Now to break our ties with the monarchy, that is something we could do unanimously, either by adopting our own monarch or scraping the monarchy altogether. Short of that though, unanimous consent of all Commonwealth Realms would be essential to make any change to the rules of accession such as establishing an elective monarchy.
Also, seeing that citizens of a Commonwealth realm are subjects of the monarch, this would mean that implicitly, if we wanted to share a common citizenship with a sovereign Quebec and we kept the monarchy, they'd have to accept the monarch too on some level, even if only symbolically. But again, it's something Quebec might be able to accept in exchange for so much sovereignty along with guaranteed access to such a large North American market.