Is there a universal morality that transcends nations/cultures etc. that can be enforced by an international body?
I don't think so.
For example, the Universal Declaration of Human Rights declares everyone has these, but I ask:
Don't you mean that it declares people
should have those?
Thinking / reasoning humans.
Humans
Humans
Laws, designed and voted into effect by humans.
And what if a nation disagrees?
Plenty do. And they tend to be hellholes where oppression and misery reigns.
If a nation democratically elects or socially acknowledges a leader/emperor etc. and this government applies laws that go against any of these human rights, what gives an international body the right to intervene and,
Plenty nations on earth whipe their behinds with human rights. I'm not seeing them being invaded and forced to respect them.
However, that doesn't stop the more civilized nations on this planet to judge those nations for it and treat them accordingly. For example through trade embargo's etc. I think that's fair.
more importantly, on what basis can universal morals and rights be said to exist?
Reason.
Are they not just a creation of the more powerful nations and bodies at the time they're created?
Their creation is a result of looking at the world as well as history and reasoning about how to ensure people get to live in peace and harmony with one another, where individuals have freedom and can live secure prosperous lives in whatever way they see fit.
Or is there some suggestion of a higher, non-temporal, non-contextual, non-situational set of values that exist for all time?
I don't think so, no.
However, I do think that if we are going to agree on certain goals concerning well-being, freedom, health, safety, literacy, etc... and agree that those things are important, then we will oftenly and naturally reach similar conclusions and end up with things like universal human rights.
There are only so many paths we can objectively take to ensure a maximised potential of achieving those goals.
If so, how are these proven and what gives anyone the right to impose them on non-compliant nations and cultures?
I don't think anyone is trying to "impose" them.
At best, one is trying to motivate nations to respect them.
Again, plenty of nations don't and haven't even signed any declarations that they will be respecting those rights.
Those nations haven't been invaded to have these things "imposed" on them, do they?
As for how we "prove" that they work... well, just look around.
Look at how life looks like for average Joe in a country that respects them as opposed to a country that doesn't.
Which life seems most desirable to you?
Look at the more objective things like societal health indexes, infant mortality rates, amount of freedom people have, etc.
I think you'll find that the more a country respects things like human rights, the better these numbers and statistics tend to be.
And the opposite: the less they are respected, the worse life in those countries is. There will be a lot more overall suffering.