IF-2016: would it be allowed to use the Research and Training budget to hire a PhD student or Post Doc to do a portion of the work of the fellow on an IF?

As a general rule, the default answer to this question is NO. Category B.1 ("Research, training and networking costs") of the budget is meant to facilitate the researcher's attendance to meetings, conferences, training actions, courses, pay his lab material, books, conference abstracts, library records, publications costs, etc... The financial contribution is therefore primarily intended for the fellow and it shouldn't be used to finance the salary of other persons.
However, it's also true that beneficiaries sometimes address this kind of requests to the REA. This is discussed on a case-by-case basis by the PO in charge and our legal officers, and in some exceptional cases they might get a favourable answer, when the work to be done by this third person is rather concrete, well defined and proves to be essential for the successful development of the action. By no means, they should be hired to perform the researcher's work but to assist him/her in achieving a particular task. The request must also be supported by the researcher himself/herself and the host institution must guarantee that this use of the money will not be detrimental for the researcher in terms of conference attendance, publication of papers, training fees, etc.