Really interesting. So much divergence from the original plot that characters are bound to be OOC, but I can see that if Severus and Harry had been in this situation, they might well be like this. Which is as good as you get for AR.
I very much liked this. (And am in awe of the fact that you did that massive research, but didn't let all the information you had overbalance the story, even though building the bridge is the focus of their lives. You made wonderful use of the research. Good work!) The relationship you created between Severus and Albus, then Albus and Harry and Severus, plus Severus's back story, are completely different from canon, which gives this story an independence, an authenticity, it wouldn't have if your AR had mirrored canon in any way.