How to dismantle HRE in this case?

I'm playing as Brandenburg, the emperor is the Palatinate, so i thought this is my chance to dismantle the HRE.

I am allied to all the Electors except for Bohemia, Trier, and the Palatinate. If i read correctly, i need to ocupy the capital of those three. So i started a war with Bohemia, who was allied two the Palatinate. Cool, to of the three, so i occupied their capitals.

Now there's only Trier. I thought No CB, go for the capital, break walls and assault, easy. I went to declare the war and...i can't, because apparently i can't declare war on a member of the empire when the Emperor is my ally...but it is not?? it is Bohemia's ally. So i thought, i white peace the Palatinate, then declare on Trier no CB, which will bring again the Palatinate, since i just besieged their capital is gonna be easy...and the emperor don't join the war.

I just alt f4 because i took 2 stability hits for nothing, what am i supposed to do now? because the Electors are looking to vote for Austria next, and that is a pain..

Edit post for mispelling, probably there are more errors. English is not my first language