There was a material breach of treaty, Pakistan accepted something that wasn't in the treaty(parallel dispute resolution), it now has to accept something else that's not in the treaty (abeyance).
India is not bound by any treaty anymore so it has the rights on the waters being an upper riparian country. Ofcourse it will be fair usage of water as India is a responsible country

There's nothing Pakistan can do apart from making noise over it, which again will be mostly for domestic consumption rather than being anything substantive.