My question is simply about entitlement to benefit, however, as you ask, Partner A has a significant mental health issue and lives in rented accommodation.
The FMH would be sold when kids are 18 to release equity.
If Partner A is unable to claim UC due to equity in the FMH (which they cant actually
live in) then they will need some of the equity released so that they can afford to pay rent and live while unable to work.
I think this may be a question for a benefits specialist. Maybe the CAB?
Housing need is one of the main considerations for any settlement alongside income. The needs of both parties and the children have to be taken into account. Where possible both parties should be adequately housed.
Has Partner A been told they cannot claim UC? Are they entitled to health related benefits? Do they have a support worker who could help them?