Re: grammar and sentence length, you're right, there's no use stressing, however there are some ways to minimise this. Go through some of your practice pieces and see if you can identify 'triggers.' For instance, sentence length is probably an issue when you're using six or seven commas in a sentence without realising. Another common one is to have sentence fragments, eg. 'Engendering readers' support for this issue through the author's use of emotive language.' In most of these cases, beginning sentences with verbs like 'engendering' 'suggesting' 'implying' etc. usually (but not always) leads to fragments.
These are the sorts of things that you can actively look out for as you're writing.
Alternatively, you can spend whatever editing time you have (after each essay, or at the end when it's all done) scanning your pages for these triggers. This is usually a better use of your time than thoroughly rereading everything, since you might be tempted to alter a major idea or argument at the last minute, and chances are this will just cause more confusion, whereas having an extra 60 seconds to add in some extra full stops to break up the run-ons is generally a much more productive option.
With regards the the 'How'/ structure-based questions, yes it's perfectly fine to know your strengths lie elsewhere; most people find the structure questions tricky. However, I would recommend writing on at least one or two, since even though you may not choose a structural question (or there may not even be one) it can still inform your writing, or be used as supplementary evidence in your body paras. This is also usually some of the most sophisticated and detailed evidence you can use, as it shows the assessors you're capable of seeing that a text is more than just the sum of its characters and themes.
So perhaps plan some evidence out if you don't have the time to write a full essay, as it can't hurt to have a bit more evidence up your sleeve, but like the syntactical issues, it's not worth losing sleep over.
↓ IMPORTANT FOR ANYONE CONCERNED ABOUT STRUCTURAL PROMPTS ↓
Something I've noticed going over VCAA documents and what not... (me and my pathetic Saturday nights) VCAA are aware that 'How' prompts are incredibly unpopular, and they've been a little devious sometimes.
Random example from the 2013 paper:
i. How does Cat's Eye show the importance of family in personal development?
ii. Cat's Eye shows how difficult it can be for people to come to terms with their past. Discuss.
Even without having studied the text, you can assume family is an easier theme to talk about compared to time, the past, and 'coming to terms' with things. In this sense, VCAA have started giving you rather tricky character/theme/V&V prompts, and then a relatively manageable structural one alongside it.
Another thing you might notice about these prompts is that the second one could easily be reworded:
ii. How does Cat's Eye show that it can be difficult for people to come to terms with their past.
VCAA have never written two prompts that begin with 'How' for the same text on one paper. But they can disguise a prompt quite easily.
And you people think I'm evil...