I'd definitely recommend taking the NQE a year earlier. My school didn't allow it, which is a bit of a shame - I would've liked to have gone to both physics and chemistry. The only NQE I didn't take was biology, so I'm not an expert in regards to it, but I do have some friends who got in without taking any biology whatsoever, so the fact that you're doing 1/2 shouldn't matter too much. There are always some people who go a year early - I had a friend from physics (who also went to maths camp from year 8 onwards) who went to biology in year 11 (after passing the NQE in year 10), and in chemistry this year there were 2 students from the class of 2011 (I think...).
In regards to how good you have to be to get into the summer school, it depends. They invite 24 students each year to each science, but they always drop beyond the 24 rank and usually into the low-mid 30s because quite a few people qualify for more than one science (I think we had 5 in physics), can't go or don't want to risk being ruled out for maths (you can't do both). I honestly don't know what sort of marks you'd need on biology NQEs to get in - I'd presume roughly 80%, but it'd fluctuate from year to year - the style of paper is now more conceptual and less factual, so the past papers aren't too indicative. You can do quite poorly in chemistry and still get in - low 40%s to high 30%s is often enough (but often fluctuates) - I just qualified for chemistry (I did so much better in physics that I didn't find out about my chemistry qualification until the camp) with mid 40%s, so anything above that should be fine. Then again, last year was a really hard paper... I'm not too sure about physics, because there'd be a huge range of scores - I'd say about 50-60% would be the threshold, perhaps lower if it's a harder paper (and much, much higher if it's easier).