I'd hopefully be far better in sketching this out in person; it's incredibly difficult to describe a sort of framework like this over the Internet
With regards to the first bit, I am suggesting the 6 units are taught over Years 10 and 11, with 360 hours or so recommended teaching hours, not on 3 hours a week- more like an hour a day on average.
In response to the second bit, I'd like the opportunity of a challenging, 2 consecutive AS difficulty Maths courses to be afforded to KS4 students who would benefit from it/could cope, hence why I suggested limiting it to the top quartile or so of students. My argument is that with a weaker student base at A Level, these results are achieved, so with a far more restricted KS4 student base (mathematical maturity notwithstanding), similar results *could* be achieved. Even an E grade would be something to be immensely proud of, due to the student's enhanced skills compared to those similar to them in the past who were artificially capped at an A* at GCSE. Those who got less than a B could continue studying it in Year 12 and maybe get on to some Further Maths, where the average modules would be far 'harder' than present.