This is a pretty common issue and I should probably add that in to my guide but I seem to be lacking the time to do so…
Anyway, summarized version is, on the default browser of the device that you play ABOW on, go to http://mbga.jp and you should be logged in to the same user as the one attached to your game. You have to find out how to “activate” your account where you can input stuff like name, e-mail, a password, etc (I forgot what buttons to press to do this). Once you do, simply go to your e-mail and activate.
When transferring to a different device, make sure not to press the big red button on the front page of ABOW, but the smaller silver button, which will prompt you to log-in with your mobage account. Use the e-mail and password you registered when you activated your account.
That’s a pretty nice idea, and I think if they do that, they should probably scale the amount of XP you get after the event. It’d suck if you spent 50 AP to only get 30 AP worth of XP haha.
As for updates with my progress in the event… I am progressing pretty terribly. Sure I got the Naoi SR and Kanade R pretty easily, but it took me 5 days to get even the Yui SR >_< gonna try for the Yuri SSR and halloween event if I happen to play at the right time