The lower contrast looks better!
I wouldn't worry so much about the exact palette at this stage, since these are placeholders. The actual colours you'll want for specific walls will differ. You will have details and other features to help you show the planes of the walls, and you'll probably have less of a need to distinguish things like bevels from the vertical and horizontal planes, since not all of those distinctions are important on every kind of tile or in every environment.
With the overlapping, you could just make it so that characters can't walk behind the walls. Make it so that characters are always walking closer to the far wall, so their head and upper body always sticks out. It might not be "realistic" but many games do this and it works quite well. It works especially well if rather than just having nonsense blocked elements immediately behind the wall, the player
can start walking behind the wall, but is gently pushed upward even as they only press left or right. This is seamless if the game employs this kind of smooth collision resolution in general, where characters round their way around obstacles automatically. Not ideal for an action game, but great in games where precisely controlled movement isn't needed.
Level design nitpick: When making horizontal hallways, don't let the top of the south wall line up with the bottom of the north wall, as tangents like that create ambiguity. Either have the floor visible, or let the south wall overlap the north wall:
Here, the overlap still creates a tangent because of how the top bevel lines up with the bottom part. More overlap makes it more unambiguous, but this makes a very narrow hallway:
So, I think you'd be better off having the hallways wide. This has the benefit of also having a visible floor in them, which will make it very clear to players that the area is navigable.