Assume that the kill-plaugue is released, it'll hurt them WAY more than it'll hurt us. Win-win.
So, last thought on dwarf populations: we already have a book covering one solution: we know chaos dwarves have restored a population at least as late as the Western dwarves twice in the OOC timeline, from a very small base. Therefore, if the difference is biological, we'll find out in the skaven anatomy book. If there isn't anything there, then cultural approaches are much more likely to succeed.
Basically women are the bottleneck here- it really doesn't matter in the end of you lose 4/5 of all men to war as long as polyandry is culturally acceptable (yes, per WoG) but only having one in four live births contribute to the next generation is crippling. It would lead to the actual pattern we see too: slow grinding attrition that can last for centuries but does not meaningfully impact dwarf army numbers, combined with massive irrecoverable population drops when Karags fall and civilians get massacred. More babies overall would help, but a shift to 2-1 instead of 3-1 male/female births would allow compounding forward as the portion of the population which reproduces gets larger.