The problem: you agree with another kingdom to have a nice battle, the result, you give that kingdom the button, and they then turn on fortified mode and start casting meteor showers and other nasty things before actually starting a war, for several real time days. This happens to us on 50% of all wars. And i personally think its a gamespoiler.
As a result of such a not many kingdoms want to give the other kingdom the button creating quite a few annoying situations.
So i would suggest:
A option to have a agreed war button, both kingdom leaders press the button and at a certain point war starts and both kingdoms go wild, ill leave the exact details of how/when it starts open. When 1 of the kingdom leaders has pressed the button, he can cancel it if he wants to, when both did, the war is agreed on and starts at a certain point.
The normal war-start rules should off course stay in so when 1 off the kingdoms is looking for a advantage by acting prematurely the other kingdom can start the war like normal.