Just putting in my two cents; it's important to remember that sample commentaries generally are not written under exam conditions (from what I can tell/have heard). Sample commentaries (like the ones in the Green Book, for example!) are written to cover everything the author can find in the text, so that a commentary written under a more limited time is guaranteed to either find their features and points in the sample answer, or to know that they were wrong in their assumptions.
What you choose to make its own paragraph should depend on how much you have to say about the feature.
If you can write a whole paragraph on one feature (firstly, kudos to you, because that's crazy!), go for it.
If you'd rather write a general 'lexis' paragraph and name a bunch of features as they link back in similar ways to the contextual factors, go for it. Plan for what you can find, rather than what you think you should write according to a stiff structure.
Generally, though, it's a really good idea to do all the contextual features as your first paragraph, because then you have something to link back all your subsystems to. You don't have to, but I find it really useful.
