I can't really comment on the mechanics question -- not that familiar with the details, but I'm so glad you brought up the rest of this topic. It's something we think about often.

Even without the asynchrony you mention, I think there are many reasons to try to avoid early college. Every family and situation is different and I know early college works for many, but we are also facing running out of school math and yet (at the moment) hoping for college at a fairly typical age. I agree that university math can work but can also have issues. I think my son gets access to better thinking and more love of math in the math circle than in our local colleges. If local college options were better, that might not be true.

My kid hasn't found much "school math" to be useful and has consistently been frustrated, so we've tried to go outside the standard curriculum as much as possible. AoPS is an obvious solution and taking all those courses takes time. Despite hating repetition, I don't believe DS learned everything possible about geometry at 10, so Olympiad geometry, or more contest math, or more discrete math are all possibilities. I think repetition with more depth is very different that repetition of school type math, which he would find excruciating.

Math summer programs in the US usually cover math outside the traditional curriculum (Mathpath for middle schoolers or Awesome math, or for high school, Promys, HCSSiM, etc.). These are expensive but also many are international and provide access to higher math and math growth in the summer at least.

I think it's been useful to focus on relative weaknesses in contest math, so speed and calculation have been drilled in math counts and been useful without increasing the level of math. I can see this frustrating a kid who really wanted to win, but it's been a radically different approach to math for my kid and useful, even though he likes thinking about a problem for days rather than one minute. While I think it focuses more on what I see as "tricks" than as deeper math, it's definitely been a learning experience for DS.

The wealth of online free options currently amazes me. If you aren't looking for credit but only growth, much of the math curriculum at some great universities is online. MIT OCW or coursera, udacity, etc. offer options.

We are trying to use the extra time to explore "mathy" topics more deeply that fall outside traditional math courses -- physics, economics, computer science -- can use math and apply it without being a typical math course.

A math circle has been huge for my son. IME, math professors running math circles are absolutely uninterested in age, or even in true readiness. I think my son entered a higher level circle before he was ready, but he was happy and engaged and so welcomed. He has grown to belong there. That exposure goes way beyond any curriculum and has fueled interest in math more than anything else. The people running it are dedicated professors who love math, love smart kids, and love feeding the fire.

We've also considered boarding school, but I'm quite ambivalent about it. Places like Exeter offer math far beyond typical high schools and I suspect such places exist in the UK??

I'm trying to focus on feeding the love of math and other subjects without worrying much about credit. DS reads a great deal rather than sitting through a formal course. He's self-taught a lot of programming in his free time. He's often frustrated with the formal course due to pace, so this has worked better for us. If you need credit, I think it gets harder.