I am an American student currently residing in California. I have been offered acceptance to the above universities as well as: Aberystwyth University, University of Exeter, and University of Kent for IR. I have listed Durham, Essex, and QMUL as my top three options as I am attracted to their course modules. However, I would like more of an informed opinion regarding future employment opportunities. Which university offers more expertise in the field of politics for postgraduate study? Which university would be worth the international fees? If I were to find work in my field in the USA, which university has more of an international reputation?

Essex has the strongest Politics department in terms of the research rankings, and of all the UK's Politics departments, it's probably the closest to US style political science with a lot of expertise in quantitative research. If you are even considering a US PhD programme afterwards, that's the choice that will have name recognition in US academia. Aberystwyth is the other well-known name in US academia for IR (historically one of the first places to teach IR) but generally for the opposite reason - it is known for post-positivist work and as you'll know that's less popular in the US, but makes Aber attractive for those students looking for something different.
As you've got Exeter and Durham on your list, I'm guessing you are interested in Middle East politics. If that is the case, I think employers in the US are going to be interested in language abilities and experience in those countries ahead of university name, as I've seen enough articles bemoaning the lack of people with those skill-sets. Outside academia, I don't think any of your choices are going to be well-known in the US. Durham is the most prestigious university in the UK on your list but that doesn't necessarily translate to US name recognition. Out of your choices, I think I'd go for Essex if a PhD is a possibility and probably Durham if it's Middle East politics driving your interest. QMUL offers internship possibilities but Paris is very expensive for housing, and the module options are limited (although perfectly fine - the only one of your 6 I'd avoid is Kent).