> if we go  too low, there is the risk of developing resistant mites quickly 
> due to  killing all but the very most hardy which then multiply and 
> interbreed
>
> If numbers are kept low, what is the likelihood of them  interbreeding?

I don't understand the question.

Varroa mites do not need to interbreed to reproduce.  They only interbreed 
AFAIK when two foundresses or more happen to enter a cell with a pupa which 
survives, uncapped, to emergence.  In that case, any male offspring can mate 
with any of the females which subsequently mature enough, forming a mixed 
family.

Otherwise, it's all in the family and the inbred offspring are as viable 
AFAIK as the offspring of interbreeding. 

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