Part of the question was about the rules-based international order, which is more of a political concept than a strict legal one. Many countries disagree what exactly it means. There is no rulebook which says, "if you didn't break any of these enumerated rules lately, you will be a trusted member of the international order." Pakistan has had military dictatorships, deals with North Korea, and unstable tribal areas. For some decisionmakers, religion may also play a role, but I believe it is less relevant than those three. India is perceived as more democratic and stable, which might underestimate their nationalists.
As to rules banning (or trying to ban) missile development:
- The UNSC has passed resolutions banning specific countries from developing certain weapons, e.g. North Korea. I'm uneasy with the legitimacy of those -- the UN is no world government.
- Various treaties ban the development of nuclear weapons, which is sometimes interpreted to include delivery systems. But look at the list of the non-member states.
- Various countries have agreed on the Missile Technology Control Regime. That's simply the voluntary agreement of certain countries not to sell missile technology to some others. This doesn't stop non-members from buying, if they can find a seller.
- Similar for the International Code of Conduct against Ballistic Missile Proliferation.
My interpretation is that "the West" -- a fuzzy concept -- strongly wishes to discourage Pakistan from developing strategic ballistic missiles because that is seen as destabilizing. There is no international law that "the West" must sell technology to Pakistan, or trade with people who do sell technology to Pakistan. There is a fundamental difference between a blockade (an act of war) and an embargo or multilateral sanctions (just trade policy, unless they go against trade and investor protection treaties a country might have signed).
"The West" also wishes to discourage India from developing strategic ballistic missiles, but that is seen as much less urgent because Indian missiles are seen as less destabilizing. Seen from the Pakistani side of a bitter conflict, that appears unfair. Seen from a western capital, less so.