Perhaps don't navigate the email conversation at all, but ask for a call instead.
That is under the assumption that you don't decline the offer immediately, and want to try working out some some solution that might be satisfying for both sides. There are serious reasons to simply reject the offer, but other answers covered that.
Startups are generally very dynamic environments, so for questions like "will you need another iOS engineer in 12 months" the most honest answer is likely to be "I don't know", and crafting anything more precise in email is a lot of work. Direct conversations allows much more granularity, and way more chances for mutual understanding.
A call may give you an opportunity to explain your position and goals easier than an email, and you'll get much more clues about the intention of the other side, because you'll see the reactions, not just dry text. The CTO (or whoever you talk to) may also be way more open talking about details than in writing. Like, what development of the team they expect - do they plan to have open positions that you might transfer to? What exactly would be your role in QA and how would you work with the other people? Take it with a grain of salt though, especially if the other person agrees to easily with suggestions like working part time QA, part time on features - that might mean they just didn't think it through.
In any case be very upfront about "QA is not a role I am looking for" and that any cooperation is under the condition of working out some sort of compromise.
One possible benefit is that it may turn out you both agree that this position is not such a good fit after all, in which case you'll give it up with no regets. Perhaps.