IOF in the USA & taxes

Hi everyone, I started my Marie Curie IOF individual postdoc in October 2014 and for the first i.e. outgoing phase I'm in the USA at UC Berkeley for 18 months, and in 2016 I will go to Cambridge in the UK for 12 months of the return phase. Apart from contacting other MC scholars in the US, which I will try to do through the Alumni directory, I'd like to add to the debate on taxes. The IOF situation seems particularly complex, as it is not clear which taxes and in which country should be paid by a J-1 visa research scholar. My return host i.e. the University of Cambridge (UK) has transferred the management of my payroll during the outgoing phase to its US branch, Cambridge University Press USA, as a result of which during this phase I am subject to the US taxation. So far 37.5 % of my gross salary (both living and mobility allowances) have been withheld for taxes each month, including: federal income tax, social security tax, medicare tax, state income tax, state unemployment insurance, state disability insurance, and university postdoc health benefits. The tax consultants disagree as to whether as a new J-1 i.e. non-resident visa holder I should be paying all this, also because with this visa by the US tax system I am treated as single although I am married and I am here with my J-2 spouse. But importantly, during the outgoing phase I will not be paying any taxes in the UK. In any case, all these numbers will only be confirmed in spring when I file the tax return forms with the help of a konwledgeable tax consultant. I was wondering whether transferring my payroll from the UK to the US was the right decision, taken by the HR at Cambridge, but I guess it could work in my interest as in the UK I could be paying even higher taxes - please correct me if you are knowledgeable about this. As the university does not give any official advice on taxes, and as the HR departments of both my hosts haven't been helpful nor responsive in this respect, I have already emailed and visited numerous offices and been to two different private tax consultants who gave me two different interpretations of this situation, and so I've been dedicating the precious time to further paperwork and searching another consultant. I'd rather save time and just leave it like this, but I suppose it's important to check it well now on time so as to avoid excessive payments or problems when filing the tax return forms in spring. If any of you have had a similar experience in the US, I'd be happy to hear from you. Although I am starting my participation in the MCAA by these financial issues, please don't be deterred as apart from that I would be very interested in contacting other IOF postdocs and scholars in the US just to share the experiences and knowledge and build the community. Marcin