Deep breaths OP.
It all feels overwhelming right now, but there is really only 1 item that has real urgency - the remortgage.
Are you able to arrest the application & withdraw without penalty?
That would give you breathing space to assess whether the extension is worth doing now. The fact that he is threatening to move abroad is double-edged. If you want to divorce over this, it's positive, as he may be considering leaving you & DC in the marital home, given his remark about affording 2 homes.
You have PP to turn to here, & you must make finding a great divorce lawyer a priority. Get copies of all financial into like salary, pension, shares, savings, mortgage & any other assets. When you have a lawyer, store them & yours & the DC passports, at their office.
These first 2 steps put you back into a little more control, because knowledge is power. When your solicitor is able to review the asset spread, she can give you an indication of what your share could be. That, with CMS & possibly single parent benefits, could be enough to secure your freedom. But some big decisions need to be made, so keep posting here for support. 