I don't know anything about Mandevilla, but it ought to work. First you cross M. laxa with another Mandevilla. Seeds are collected.

The resulting hybrids are grown, and self-pollinated. Seeds are collected.

Now this generation of plants can be tested for cold tolerance. You would want to expose them to enough cold to kill off most, but not all, of the plants.

Hopefully, from these plants you can further develop the cold tolerance and other traits that you want.

does anyone know how to cross pollinate mandevillas?

I love Mandevilla laxa, and faaaaaar more than any other Mandevilla I have ever met.

Is it the only one with scent?

Years ago we had one growing up a brick wall, until Eucalptus nicholii decided to self prune one dark and stormy night. It knocked down the brick wall and the Mandevilla disappeared. And who can blame it? It would have been terrified.

I collected seed from it one year, sowed it, produced a tray full of seedlings which I gave away to a friend who grew plants to sell. I never realised that one day I would lose my precious plant.

And Daniel, I notice you live in Australia! How victorious is that?

Oh, I just realised that your post was rather long ago. Perhaps your Mandevilla has got up and left too.