Most of the "early war" crying is a way for people to complain that they weren't ready. Alliances that aren't flexible to the change in reset to reset politics will always have a difficult time if things don't go 100% according to plans. If you change this they will simply find something new to complain about.

If there is a change to game mechanics restricting "early warring", it shouldn't be more than 7 days OOP.

Yes: only for the first 7 days OOP.