Unfortunately premium is not support included however we are a few guys working the forums as best we can as an extra job on top of our regular jobs 🙂
I’m not sure wether I’ll be able to find anything you haven’t if you’ve already did this extensive testing but I can have a quick look.
Could you try something a bit drastic.. install a fresh WP installation in a subdomain or subfolder in the same hosting and see if the problem exists there as well.
