Yeah so I got the test footage back, cracked mirror has no effect at all on the film, it looks fine. There could be a little more flicker in the viewfinder, no big whoop. I did get a mirror (not expensive) which does match the existing cracked mirror, shouldn't be much of a problem replacing it if and when it is neccessary.
In other words, take any of these so-called "experts" advice with a grain of salt.