Thanks for the replies.
Do you happen to know the shelf life of a mixture of the pre-hardener? (Is it something that you could make a liter of it and keep for a while, or would you have to keep mixing the Formalin-3 each time you use it?)
Also, assuming that you can make the film survive the development without the pre-hardener, and assuming you are developing just the silver layers of the color film with B&W chemicals, am I correct that a Formalin-based dye stabilizer would not be needed? In this case, would a hardening fixer be enough to make the film less delicate when it is done being developed?