I realize that in a previous discussion we covered most of the questions about auto-reversing segments in DCC. And I apologize if this is redundant.
In practice, you will probably get away with longer trains with just metal wheels not connected electrically to other axles at least 99% of the time. In the remaining <1%, there may be a short reported - or not if you're lucky. If not, you won't even notice anything amiss at all. But it's still not a good idea. If you add in the system that some people use to stagger the gap between left and right rail (which I am 100% sure is pointless, but does no harm normally) then the 99% will drop significantly, in line with the length of the staggered bit.
That is certainly safe. Even one loco OF a powered consist is plenty. Shorter will work, but there is no set shortest distance. Stick to the length of your shortest loco and you'll be good (although if you push a train instead of pull things need to be looked into more carefully).
Q1 is about how one autoreverser works, whereas Q2 is about preventing two AR sections from in-fighting. It's like asking for the difference between inflation and the exchange ratio between dollar and euro. They are both about money, but there is no other relation.
That is correct. It's all about not crossing both gaps simultaneously.