“I know Oxbridge takes very few U.S. applicants, but do you think my chances would be boosted by my scores?”
For Oxbridge, scores are a hurdle: once you are over the hurdle they are not as important as the interview & their own tests (TSA, for example). Between 2 candidates who are closely matched after interview, the higher TSA score could tip it, but a 2400 won’t tip it over a 2200.
“would universities accept the credits/would employers look well upon it?”
Do you mean would a US post-grad program accept an Oxbridge undergraduate degree? absolutely (if you were applying to med school there would be some specific hurdles though). Do you mean if you transfer part-way through to a US university would they accept the credits? I don’t know, but would imagine that it would be hard to figure out how to line them up, as they are structured so very differently. Employers view will depend a lot on the field, and what sort of graduates they are looking for.
For Ivy league schools, as dblazer points out you are very strong in the academics, but don’t have much in the way of leadership, and it isn’t clear what you have done with your summers. A one-off summer at Governors school tracks more JHU, not HYP. The dean of admissions at Princeton said ‘We don’t care what you do during the summer- just that you do something. And we want to know not just what it has meant to you, but how you are acting on that’. She went on to say that they read many essays about community service trips to a poor country, so standing out is hard.
This from Harvard: http://www.hcs.harvard.edu/womswim/recruiting/myths.html