I’m afraid I think the emergence of a new partner for one of the parties greatly increases, rather than diminishing, the risk of complications.
What kind of stooge is the new partner, that they wouldn’t mind their partner entering into a 25-year financial commitment with the ex, on the basis that it might be terminated “in five years or so, all being well”? To my mind the new partner must either be a saint, or not been told, or only a casual relationship anyway - but if the latter, what happens if it turns serious, and they have to be told, and they DO mind?
Please, walk away! I know it’s hard, if it was your dream-home, but there will be other homes, and, hopefully, other partners.
It’s bad enough if you can’t avoid this situation, because you already held joint property. But who knowingly goes ahead with this, when they’ve already split, and the partner is with someone else? I foresee trouble ahead - for all three of you, if I’m honest. There’s a limit to how easygoing any partner of either of you is going to be about this arrangement.
And I have to share Jane’s concerns that if you wouldn’t be granted the mortgage in your own right, it’s because it’s not deemed affordable on your level of income. OK, maybe you can get a five-year fix, at historically low rates. What happens when the fix ends, and rates soar (they’re very likely to be higher then than they are now, as it would be difficult to get any lower). If interest rates doubled, assuming your circumstances were otherwise broadly the same, would you still be able to pay?
Five years is a long time with MS. It’s even a long time without MS. What happens if you get too sick to work, you get behind on payments, and your ex IS pursued for them? Really, I’m struggling to see anything positive about this for any of you, except that you get the home you want in the very short term, but you’ll always be at risk of losing it, if your ex has a hissy-fit, their new mate has a hissy-fit, or you can’t keep up repayments.