Can you doubletime and then overwhelm? Has there been an official ruling yet?

No, because both actions require a move action, and you cannot give a figure two actions in one turn.

it says you can't use any speed special abilities but double time is a proficiency.

You're missing the point that Kenntak was making.

To double-time you have to give the unit a move action. To overwhelm you have to give the unit a move action. Doing both would require you to give the same unit two move actions at one time which is not allowed.

It's the same logic that restricts the use of SA's when you Charge such as Charge/WM. They both require a close combat action to be given, meaning two actions at one time.