Look, I'm no aerodynamisist (cant even spell the word) but something things strike me as odd and I see no escape from it by the other teams is this. Actually there are 2 things.
First is you can argue that the spirit of the rules doesn't exist, but it sertainly didn't stop the FIA from ruling by claiming something against it. (aka. Ferrari flex floor, wings). So I don't think that the "umberfuser" teams, can just say there is no such thing. [-X Though I, would want to side with them and state that isn't as well. Rules are black and white and there are no spirits. I and everyone though have seen that the FIA doesn't entirely think that the spirit of the rules doesn't exist. This is another issue all together though as banning on the idea of the "spirit of the rules" is a controversial idea and alone may not be enough for a ban.
There is this issue though of..."the hole". Mainly the starter hole.
Certainly, its a very obvious issue. But the rule still says "solely". Which we all know means "for no purpose other than...X" And designing a hole to create aero effect and benefit, is a real and tangible action.
Some and teams may argue that the hole in of itself cannot do anything. But if they did, then the FIA might tell them that they think it is there for something other than "solely" purpose.
What would be the team's official response to that question?
AKA.....Dear..(insert name here) Is the hole there for the "sole" purpose of starting the motor? They will have to carefuly formulate the response not in simple concrete technical terms, but also to not take the FIA for dumba$$es.
We have seen in the past (BAR fuel tank) the FIA act less than favorably to teams blatantly misleading somebody about the function of a device.
In the case of these diffusers when those questions come up, the answers better be formulated with more than just the premise that the FIA are as black and white as the rule book appears to the reader.
And, surely an answer of..."It's there to circumvent and exploit a loophole created by the allowance of a hole for a starter and in the end generate more downforce" will not fly and spawn plenty of "spirits" to answer to.
Moreover, I doubt that the "protesting" teams, would simply take whatever response they get from the "umberfusers" and stick it right in their face with a response formulated with a "but what about....and we asked you about that before" (Red Bull/Renault).
Just a thought....