We don’t know whether the members of the Security Council are all democracies, or whether the Security Council’s veto structure fails to “provide for democracy.” Since the argument doesn’t involve either of these considerations, it can’t depend on any assumptions about them.

In other words, no other nations would become major powers. If negated—if other nations were to become major powers—the premises would support giving those other nations veto powers too. So the premises support restricting veto powers to the original five only if (B) is assumed.

Allegiances have no effect on the reasoning. The argument is that the five major powers should have permanent, sole veto power so that as peacekeepers, they can say “no” to decisions they strongly disagree with. Whether they form blocs is irrelevant.

Allegiances have no effect on the reasoning. The argument is that the five major powers should have permanent, sole veto power so that as peacekeepers, they can say “no” to decisions they strongly disagree with. Whether those vetoes protect allies is irrelevant.

Whether decisions are biased has no effect on the reasoning. The argument is that the five major powers should have permanent, sole veto power so that as peacekeepers, they can say “no” to decisions they strongly disagree with. The content or bias of any decision is irrelevant.