There should be an exemption. If a Duke is banned they should have the opportunity to secede no matter what.
That would open up a glaringly obvious loophole: want to found a new realm? Still on good terms with your government? That's OK, have them ban you anyway!
As for the 15 nobles I think 10 might have been a fairer number. Astrum was founded by 5 or 6 nobles trecking through winter and fighting the hordes to make it to Libidized.
There's a big difference between letting people found arbitrary-sized realms when the overall playerbase is healthy, and letting people found arbitrary-sized realms when the average realm size is below 20 and there are issues with finding a decent, interesting, and interactive realm wherever you go.
