First of all there was an agreement by the name of Algeria agreement in 1975 that addressed border disputes that should have been respected...
Secondly Shah as you described is the one whom saved Saddam a$$ back in 1978 by informing him of a military coup within Iraq by Soviet Union:
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So what was the problem?
Khomeini did a great job by not accepting ceasefire:
1. there was no real meaningful peace plan which in the invader, compensation and returning to international borders would have been guaranteed till 598...
2. Saddam started the war consequently if he wanted "peace" simply he could have withdraw from Iranian territories to international recognized borders not only he didn't , he stayed in Iranian territories and called for ceasefire. silly ...
3. We couldn't trust a person like Saddam whom teared off an agreement recognized by the itself and the UNSC before the cameras and then he started the war...
4. His occupation of Kuwait proved Iran stance to not trust a maniac like him who attacked his people and also Iranian by CW ...
Moreover Iran-Iraq war had nothing to do with Iran Iraq problems it was a plot hatched by American and others :
And I am telling you those who like your post are the same guys whom bankrolled Iran Iraq war and later on cheered up the US invasion of Iraq and "oil-for-food programme" causing hundred thousands of Iraqi children death during 90s ..
if you wanna team up with somebody find someone that worth it ...