i’ve never had a problem with the cream cheese getting too soft. in fact, i just posted that i recently put the entire thing over a bain marie to melt the seized chocolate, but i just remembered that once i tried to make the recipe from memory and put the ingredients together in the wrong order. the result was a delicious, white chocolate soup. nothing would make it firm up, and it was a 6x batch! could it be that you are combining the ingredients in a different way than the recipe?
FYI i saved the soup and added it to future batches which were properly made and it combined just fine.
another option is to make a regular white chocolate buttercream. it will still taste great for the cheesecake.