While I agree with you that the peacekeepers need some tweaking, I don't think your proposal works very well.
I think it was WIF 4 or 5, (I wasn't playing, but my Dad was back then), when allied players hit upon the following exploit. Suppose Germany sets up historically, and smashes Poland on the first turn, doing little else. Then, on an allied impulse, the Soviet Union declares war on Belgium. Since then, as you propose, the major power couldn't enter unless they were at war with someone at war with the minor, Germany effectively couldn't attack France, unless they could break the garrison, DoW the Soviets, and fight a war on two fronts.
What we used to house-rule is that if the minor was conquered, and the peacekeeper Major Power wasn't at war with the conquering major power, they would have to leave. It had the biggest effect on Persia, since the Japanese Peacekeepers rarely left the coast, but if you pounded Tehran……
Anyway, hope you find this helpful.