But honestly, what is really going to happen?
1) The property has to be unwound: a shame about the costs, but really not a life drama
2) The relationship ends, the house has to be sold: a shame about the possible loss, but not a life drama.
3) external circumstances change for the worse: house has to be sold, a shame about the possible loss. Debt remains. Either personal bankruptcy or keep paying off and live frugally: bad, but also not a huge drama.
The arrogance here, with which 2- or 3-room apartments are spoken of, is quite telling – as if living in an apartment were an unworthy punishment. For most it is probably not such a nice prospect, but therefore taking one’s own exaggerated ideas, which really are not realistic, as a basis for all kinds of fear scenarios is not so helpful.
The decision from the OP depends on him and his risk tolerance. One person would certainly dare in a comparable situation, another definitely not.
Anyway, the consequences in the worst case are indeed tough, but also not life-determining.