Good questions!
I guess that's a good call on the reproduction guidelines. It's incredibly open-ended so that people can make pretty much any kind of creature that they like, but I understand there's also room for abuse. I think what I'll do is put in a minimum gestation period based on size category; 1 month, with an additional four months for every size category above Tiny. That gives us mice with 1 month gestation, humans with 9 month gestation, and elephants with 17 month gestation. Not perfect (elephants are 22 and mice are about 20 days) but close enough to satisfy me.
The other main point I'll put in is that the creature can't reproduce until reaching adulthood.
As for the doubling up on grafts, you are allowed to have multiple of the same graft if they occupy separate body slots normally, such as claws in more places than normal. Since these growth slots can count as any body slot, you can double up whatever you like. Nothing particularly unbalancing about having exceptionally powerful death throes, especially from a 4th level graft.
Glad you like it!
Yes. It's expensive stuff. You can always transmute a smaller amount if you want. But at normal xenoalchemy rates, at Doctorate-level you can make about enough for that in 6 months to a year. As some people pointed out to me, orichalcum is pretty much the biggest gamebreaker in the entire system.You know that one cubic foot of platinum costs 665000 gp, right?