1. Is it possible to switch from the MPhil to MSc or vice versa at all once the course has been embarked upon? I.e. by starting on the MPhil but then opting to write the shorter MSc thesis and subsequently progress directly to DPhil after 1 year.
I'm not aware of anyone ever having done this. While it's true that the bulk of the 1st year MPhil and MSc courses are identical, MSc students will be working on their theses throughout the first year in a way MPhil students will not.
2. With MPhil I gather it takes a 65+ average and a distinction in the dissertation to progress to DPhil, however for MSc it seems less stringent (cf. DPIR FAQ). Is that correct? Are these requirements rigid? I generally perform extremely well in coursework but less well in exams and so I'm worried the exam-basis of the MPhil might set me back.
I wouldn't say it's less stringent. It's true that there's only one exam in the MSc and the rest of the assessment is by coursework, but in both cases the thesis is the most important element of the application. But given that you're already doing a Masters, and given that it looks like you have a thesis subject in mind, there's an obvious prima facie case for doing the MSc rather than the MPhil if you want to minimise your exposure to exams.
3. Could I apply to both the MSc/MPhil and DPhil and if I get in but don't get AHRC funding for the DPhil pursue the MSc and then reapply for AHRC PhD funding when progressing to the DPhil?
I think so. 4. Will it harm my chances of AHRC funding for the DPhil if I do a degree in between my AHRC-funded MA and the (potentially AHRC-funded) DPhil?
No idea!