Uggh, sry to see it happen to such beautiful paint jobs. Humidity frosting is a real thing; if you live in a clime like mine you wait for those few days you can spray and capitalize. Yes, brush on varnish over the top can “tone it down”, but if you find a true fix please let us know.
Again, sincere condolences 
I believe this is called “frosting”?
I had something similar happen to a piece of terrain, but far less extreme. I only had whitish spots all over and I was able to get rid of them with a brush and a q-tip soaked in nail-polish-remover and some gentle scrubbing and brushing.
I doubt that’s going to work here, but you should at least try it out on one model.
Alas, I repainted my miniatures (it was for Arcadia quest) it’s the best way I found.
I have heard that brush painting gloss varnish and following it up with another matte varnish can fix this. But I can’t say I’ve ever used it.
Good luck, hope you can bring them back 
Splendid! Wonderful that you were able to save them!
Something else you have to be aware of using spray varnish, no matter the weather, is to spray the miniatures and then glue on tufts, because the spray will frost the tufts even if the model is unharmed. The capillary action in the tufts attracts the spray into droplets, like dew on grass sometimes.
I was using Citadel matte varnish and then started using The Army Painter Matte varnish and I had some frosting on the last batch of mini’s I painted. I painted them with Vallejo Matte with a brush and it fixed it on most of them, but some were still blah, so I went over the frosted areas with The Army Painter Soft Tone and they looked satisfactory.
Humidity frosting is a real issue in many climates. One thing to note is that not all spray on brands have the same tolerances. For instance spaying GW brand in anything over 40-50% humidity is asking for disaster but Testors can be safely sprayed in up to 90% humidity, with most other brands falling somewhere in-between those extremes.
One last tip, since I have some experience with this issue, is regardless of the brand’s tolerance of relative humidity, spray outside and immediately bring them in to dry, even between coats. Although the greatest potential for problems is in the spray (has something to do with the expansion as the spray leaves the pressurized can) it can still absorb more to the point of issues just sitting in it. Glad you were able to get these guys back in action!
Thanks, I’m sure that was the cause of the frosting as it was the first time I left models outside overnight. Lesson learned 
I’ve also found it’s really important to shake the can well before spraying (as in, look at a clock and time yourself shaking for at least 90 seconds)
The white - opacity can be caused by two main factors.
To fix as you say, often can heavy layer of gloss or another matt coat over the top can re-activate the chemicals in the opaque layer and allow them to re-set effectively and hopefully clear or more translucent than they were before.
Extreme cold can also cause the problem. If the spray can is below about Celsius it can cause the carrier liquid to thicken and not mix evenly.
Really? I’ve varnished dozens of models with tufts on and it has never happened to me.
Really glad you have saved the miniatures 
I was going to type about trying the different varnish over the top, which has saved miniatures for me in the past (although I have never had any that looked that bad!)
These days I apply varnish by brush, with small scale forces it doesn’t take that much time and losing a squad or two to a bad varnish is one of the worst feelings in the modelling hobby!
I’ve got liquid varnish but haven’t tried it as the spray was always more convenient. I’ll give it a go at some point. I have sprayed other models since then without the same problem so I put it down to leaving it outside over night.
