Would this make my PS seem "off-topic"?

I think people are forgetting the huge difference between an 'unfocused' PS and 'more than one subject' PS.

As you seem genuinely interested in your subjects and the ways in which they link, this seems like the foundations for a really really tailored, interesting and different (in a good way!) PS, which would be far more interesting than a bland 'I want to study psychology, only psychology, I never think about how psychology affects social science/what affects psychology, I just want to learn about the brain and our nerves' PS.

However, if you have no life then maybe don't apply to Durham - they probably wouldn't reject you because of it, but they do say on their website that they are looking for applicants who can contribute fully to Durham. That doesn't mean you have to be interested in music and sport and art and drama etc. but having something/things you are interested in and that develop your transferable skills is never ever bad - and also it's vital for jobs, all the questions on job applications are about teamwork, communication, leadership, etc.

Don't forget the PS is just ONE aspect of your ucas form!
A-level predicted grades, AS grades, gcse grades, reference too...

In most of the university threads, people are panicking about having one A* at gcse fewer than other candidates, not their PS!

As I've said before, no 2 university courses are the same, and a very large number of people apply for different titled courses at different unis - that could be because unis vary in their naming (eg for courses like sport), because not all universities offer that combination, or because some universities' single honours courses are very specific, so they want to apply for a joint course at that university. UNIVERSITIES KNOW AND EXPECT THIS!

For example, only 2 universities in England offer anthropology courses which cover biological and social anthropology, so applicants for those courses clearly have to apply for straight biological or straight social anthropology courses too - I very very very much doubt that everyone who did got automatically rejected for the straight courses!

Sociology courses tend to need lower grades anyway, so if you have 3 A* predictions then I think they'd mainly use your PS to check that you know what sociology is, etc. before giving you an offer, rather than going through every word of it to see if it could relate to another subject or not! Definitely email or ring them up - only way to find out!