Wrt lesson length... DD (just completed Y7) has found that fine, including Wednesday when it was 2 hours of maths followed by an hour and a half of Spanish. They get a 15 minute break in the middle of the double and another 15 mins between subject changeover. Just 5 mins in the middle of Spanish though, iirc. It was a big adjustment at the start of the year, as she'd gone from home ed to this, but she did adjust quickly and it's been fine.
I can't speak highly enough of Interhigh - it's been the perfect answer for us. It doesn't work for everyone though; DD's best friend started at the same time and dropped out by the first half term. She was autonomously home edded, though, and just didn't enjoy or want that level of structure, concentration and homework. For us, it's the perfect middle ground between home ed and the local secondary - DD enjoys the lessons, loves the class banter and has made friends (they skype each other), and still has time to hang out with her home-ed friends (this was a big factor for me).
She works ten times better, and with more enthusiasm and diligence, for the teachers than she ever did for me, and has come on in leaps and bounds academically. She's been incredibly self-motivated with doing her homework, which I think is all helped by how visual the dashboard is - she can see her attendance, grades, feedback, what homework is outstanding, what it is and when it's due, etc. Everything is there in the lesson library if she needs to remind herself of something, too.
I don't think it necessarily works for all kids, but some children it works very well. I was just reading the thread about the 'Chinese school' programme and the low level disruption in UK classrooms... I am feeling very grateful for Interhigh, where if a child is being disruptive, the teacher can 'mute' them and ban them from the chat box for a while. Show me a teacher who wouldn't love to be able to do that! 