Use multifield values to create new nodes
I managed to solve this by swapping the Multifield for a Double Field which does play nicely with Rules.

I was then able to use Rules Action: generate a list from an integer to loop through the quantity for each category and create a new node for each, populating the required fields.

The only downside is that Double Field does not allow to populate a select list from a taxonomy vocabulary, and I could not get the Field Remove Item or Multiple Fields Remove Button modules to work with it.