Hey, so in answer to your questions..
1. yup pretty much, so a social purpose might be to.. for example, persuade the readers of a magazine to purchase the perfume being marketed whilst also provided visually entertaining stimulus
2. I've never heard of a "type of commentary" actually, but I'd interpret as equivalent to the "type of text", so yeah, like a speech or a blog, like you said

3. yup, although you can put it on a spectrum rather than saying just formal/informal if it's worth a few marks. so, for example, you could say "this text is largely informal in register, however, due to its being in the public domain, there are also some features consistent with a formal register, and this can be observed in ... (insert evidence here)
In terms of cuttings journal, blogs can be quite interesting in terms of how the writer uses language, even just keeping an eye out for interesting linguistic features in advertisements, for example, there's an ad at the moment that uses the acronym MAMIL - middle aged man in lycra, and this can show your examiner that you're really engaging in how language is being used around you
