Very good point -- to answer the questions:
1. Imagine India did not exist -- where would that leave Iran and Pakistan: they not natural rivals, and actually have supported each other during crises.
2. Imagine would Iran want to get into hostility with its nuclear armed neighbor over far away India (?) -- the Mollahs are misogynistis but not stupid.
So again:
mmm, Not so much I'd say, Chabahar:
1. is not a Natural deep sea port
-- OK you answered this -- the goals of Chabahar are rather limited
2. to have any value for India it requires a stable Afghanistan
-- still open -- either Pakistan and Iran can keep Afghanistan unstable (Iran projects power all the way to Syria!!!)
3. to have any value for Afghanistan it requires a stable Afghanistan
-- still open -- either Pakistan and Iran can keep Afghanistan unstable (Iran projects power all the way to Syria!!!)
4. to have any value for Iran it requires Chabahar not to get lapped up in the India/Pak strategic competition
-- still open (the same way Pakistan chose not to participate in the Saudi vs Iran competition)
5. Iran will still need to use Dubai because it's not a natural deep sea port
-- OK you answered this -- the goals of Chabahar are rather limited
6. The Delaram-Zaranj highway runs through the Taliban heartland
-- still open -- either Pakistan and Iran can keep Afghanistan unstable (Iran projects power all the way to Syria!!!)
So where were we?
What makes you think Iran and Pakistan are not rivals in Central Asia ? To the degree you do not see an active rivalry it is due to the fact both of them have bigger rivals to fight.