Normally I would say that your mother has the right to leave her money to whoever she chooses, but this is patently unfair.
If she wishes to make some kind of bequest to you to skirt IHT issues, that is fine and understandable, and to ensure that her family money stays with you is also understandable.
The getting your DH to make the mortgage payments when he will never own the property is outrageous, and asking you to lie to him too, that is worth of divorce right there.
She needs to look at this in another way, perhaps leaving the money to your DC, to be held in trust for them?
WRT this property, without knowing where it is, I'd urge caution, global property prices are still falling in many places, and holiday homes has got to be one of the hardest hit areas, proper research needs to be done before any money gets spent there.
I wonder, but if your relationship did ever go bits up, and this property was being paid for by him, regardless of the trustee status, I wonder if a relatively good solicitor would be able to successfully argue that as neither trustee could actually ever get a mortgage, and have paid nothing toward that mortgage, that your H paying all the instalments would have a claim on at least half of YOUR half as you are husband and wife.
If your relationship is not healthy, the last thing you all need to be doing is creating further financial entanglements.
I would also ask you giving, be honest, is your mother a friend of your marriage? or is she aiming to destroy it, cos this sure looks like a very nasty move.