Why can you only premove one at a time on phone


There's some work going on with enabling multiple premovrs in Android, but the initial step in that roll out was reversed recently. It should still be coming, though it may take some time.
I haven't heard anything about iOS

Will you be able to pre-move your opponent's moves too then the whole line gets cancelled if they break it?

Will you be able to pre-move your opponent's moves too then the whole line gets cancelled if they break it?
They'll work just like web premoves, though cancelling them will probably be different. I know the other method has been suggested before but my guess is that isn't very likely to happen.