Why Dominion status would not necessarily close Alang

If India had remained a Dominion, would the Alang shipbreaking yards still have been allowed to operate?

Probably yes, because the industry made money for India and imposed no clear cost that would give Britain reason to stop it.

India would probably retain Alang's shipbreaking industry even if it remained a British Dominion. The yards generate revenue for India without imposing an obvious cost on Britain. That incentive gives neither government much reason to prevent the work from continuing. The counterfactual therefore turns on local economic benefit rather than constitutional status alone.