Hey, to be honest, it really depend on where you are applying to. If you are applying to the UK, then they don't give **** (excuse my language) about whether you are doing double language A, if you are applying to HK, then you will look at it more. It also depends on what course you are applying to. If it is maths, then yea, they don't care about your Chinese.
If you do double language A and is ok with it, i mean grades are decent, then you can get a bilingual diploma, which might give you a bit of an advantage, but not a lot really in my opinion.
I do not think universities will reject you like that. It is an international school, so they will understand it. I live in HK, there are loads of Chinese people doing Chinese B in my school, but not sure about their passport tho, but I would imagine at some some of them do not have a non-Chinese passport.
The way to improve A2 is to read more commentaries on line and so you will know how to bull-****. Read more books so you get to understand simplified chinese. It is not that hard once you become more familiar with it.