Forward comparable console isn't a new type of consoles. New 3ds & 3ds are recent examples.
Yep. Forwards compatibility is the key to making a shorter console/upgrade cycle work. The games need to run on the new hardware without any changes from the devs. That may be a place where Xbox One's higher-level abstraction helps it. It has some low-level functions relative to the PC, but not sure if there's anything that would be a huge barrier to forwards compatibility, as long as they stay with x86. ESRAM is the only thing I can think of that could be tricky.
After three or four new generations of hardware they may have to go back to some kind of emulation, depending on how much the hardware shifts each generation.
Their VM model will put a lock on the software-side. The game runs in its own VM with the necessary underlying software, so you can have generations of VMs for running older games, just like they do with the 360.