My understanding is that:
- You(higher earner) are in FMH with younger child
- Wife((lower earner) is renting with older child and new partner
Lets ignore the New Partner for a minute - imagine she is on her own.
In this case you have 1 child to care for each (i.e. similar needs), but she has significantly lower earning capacity.
In this scenario I doubt that you could achieve the outcome of you remaining in FMH with a mesher in place which would mean that she has to wait for younger child to leave home before she gets any capital.
I would guess that the home would be sold and capital split (probably in her favour - due to her lower earning capacity).
If she got married - and the new husband could support her then maybe what you describe is possible.
If she co-habits - then i think the court would have to be cautious about the longevity of this new relationshiop and so would seek to protect her in case the new relationship breaksdown - so i really doubt that you would get an order tying her capital up in a Mesher.