I've rode past Debark probably half way to Axum and back and the road was in good shape and definitely rideable and driveable. Beyond that point can't say.
As for route from Bahir Dar to Laliblea, that's a more secondary road and if you were on a bike it's not recommended, not sure about a 4x4. A couple of friends in our group rode in and the roads are rough and chewed up their rear tires quite convincingly.
There is of course regular mini bus service into Lalibela so it's no doubt passable in regular season. Some folks at the hotel took that method of transportation in and were happy to escape with their lives, but that was mainly a function of the crazy mini bus driver, not necessarily the roads.
Lalibela should not be missed but it is easy to fly into from Bahir Dar. The road past Debark is quite scenic but the best part is for the 10km after the entrance to the park, after that the views and the road is less dramatic.
Hope that helps in some way.