From the pictures the crack has definitely grown. (And yes I do think it is a crack.) There are two main reasons a hoof wall will develop a crack and they are both related to too much pressure.
1.More pressure on the normal, healthy hoof wall than it can handle.
2.A regular amount of pressure for the healthy hoof wall, but too much for the hoof wall that is not firmly connected to the lamina.
Since she has foundered, I think the second reason is what you’re battling. The solution is simple – just relieve the extra pressure until the hoof wall grows down firmly connected to the lamina. (Added bonus: When the hooves are kept short the hoof will actually speed it’s growth up which is what your farrier is wanting.)
In regard to the hoof conditioner – hooves are supposed to be very dry (it can actually cause problems if the hooves live in a wet environment), so I don’t think I would bother using it.
Best wishes to you….