That isn't the plan, though--it would be obvious to the Gobwin Knob forces that the portal had disappeared and they'd probably be able to get word (via Wanda) to stop Parson coming through. The plan is to *let* Parson come through, then close the portal behind him so he can't get back.
How so? Wanda is in the Magic Kingdom. There is no one in Spacerock, who would be able to send a thinkagram or has an eyebook. Gobwin Knob doesn't have sending hats as well. Sure they will notice, but they won't be able to inform anyone about it.
Besides, Parson was fully aware he won't be able to go back through Magic Kingdom once they conquare Spacerock - it would stop being a capitol anyway.
In a war it doesn't matter who's right, only who's left.