Indeed, you don't understand my problem.I don't get what's your problem now, once the drivers have passed the test and you are eligible for the upgrade you can reinstall Parallels Tools.
If you still can't do it, this seems to be a very exotic situation, there are so many possibilities, that yeah, you might as well start fresh.
With or without Parallels Tools, I get this :
Is that more clear ?
I cannot install win10 with the free update process...

