Just thinking outloud re: multicast. I think it's broken. I think, given the fact that there are other mastery abilities (quick and precise) that can be taken more than one time, I think an easier method is to just allow multiple casting to be taken as many times as desired, but leave the spell's power intact.
For example, Marcus of Tytalus, I find it somewhat ridiculous that he can multicast 6 PoF, just because he has multicasting once, but a spell mastery score of 5. I think his being able to multicast 2 is reasonable. I like picking multiple casting more than once, because it really forces a character to choose between finesse and brute strength. And finesse, in this context, doesn't mean the ability finesse.
Alternatively, divide the mastery score by two, if the caster has multiple casting and round it, but always allowing at least one copy if multiple casting is selected. For example, with Marcus, and a mastery score of 5 and the multiple casting, and the Bibracte rounding rule to make it an even number, he could cast 2 copies. Both options have their merits. The first option tones down multicasting and makes spell mastery a bigger XP sink. The second option still tones it down, but it isn't as such a big XP sink.
Don't get me wrong, I like The Fixer's might stripper enhancements, too. The multicasting thing has been one of those things I've been ruminating on for a while. The idea that Marcus can flink of 6 PoF or BoAF at a squad of men is truly frightening and devastating. I can only imagine the morale hit for a mundane group of men should that ever happen. With 3, I think there's a chance, a slim chance, of holding the group together. wWth 6, I'd say not a chance... YSMV. 
Edited: due to a fat finger of typing "he would cast 3 copies" changed it to two copies.