I wouldn't worry too much about a "misdirected PS". I would agree that the PS isn't particularly important - my own interview didn't touch upon, let alone revolve around, my PS.
Regardless, you can choose to focus upon why you want to study PPE and yet not explicitly mention the subject (e.g. you state that you are interested in political economy, and why) - that is what the tutors want to know anyway. Meanwhile, that PS is equally relevant for (say) Int. Relations, and you can tweak it even further to ensure greater relevance (e.g. writer about the Washington Consensus in the context of international political economy.) Other possible topics: 1) Humanitarian intervention - obvious philosophical and political issues, with an international dimension. 2) Democracy and economic growth - a perennial favourite, which came out for my own TSA; plenty of political and economic issues to talk about, and it is quintessentially an international and comparative issue.
You get the idea. Nonetheless, I offer a caveat - if you truly want to study PPE at Oxford, give it your best shot. Tailor everything towards getting in, from PS to general preparatory reading to UCAS application (i.e. apply for PPE across the board, so you don't have to worry about an irrelevant PS). If you're confident enough in your abilities, perhaps you can choose to forgo this - but I find that the most brilliant applicants tend to sincerely want PPE and only PPE anyway.
Anyway: May I ask whether the OP (by Tom Holder) regarding the 1st Year Reading List, last updated in 2011, is still accurate? I'm using it to organize my own preparation for 1st year - i.e. what to read, what to focus upon. It's pretty clear that Holbach's The Logic Manual is the centrepiece of the logic course, for instance - but what about the ten Maths PDF files that Tom has kindly uploaded? Or the readings for politics?
Thanks in advance.