My understanding is that "ALL" of the epoxies can produce blush under certain conditions. The epoxy layer is supposed to be wiped down with I believe soap and water (I am not going to check, but each manufacturer states this in the directions or website), to remove any blush BEFORE sanding.
If you think that sanding will remove the blush, that isn't correct, it will just get ground into the surface as you sand.
The blush contains compounds which will mess with the varnish chemistry and prevents it from oxidizing to cure.
Having said all this, I don't usually wash the epoxy before varnishing and I currently have a slight tack issue as well.
If you have the shop ability, the advice is to a) increase air flow (fan) and 2) if you can raise the temp, get it up (I took mine to 82F) ... leave it for a few days, if it cures you're done, if not, you have a mess to clean up. More air and heat will help with oxidation ... hopefully aiding your varnish to cure up.
As far as thinning, Epiphanes is a high solids varnish, which should always be thinned to some degree. Initial coats are usually thinned to help setup the first layer and provide a base for the build coats that come later. Also, with less solvents the varnish will tend to skin/set too quickly and you may end not getting a smooth finish without some thinning. I agree that you wouldn't use the regime for bare wood, but I usually thin 25% on the first coat and 15-20% there after. Just makes it easier to handle.
Brian