I can't answer all your questions (I'm fairly new at this myself), but I can tell you that I'm building a solid mahogany guitar right now, and my mahogany slab looked just about identical to yours before I started working it.
I don't think your mahogany is stained. Maybe it had some sort of sealer or wax or something applied to it. Does it feel waxy?
I planed a bit off of each face of mine and the coloration is now the same on both faces.
You shouldn't have a problem building a body out of that piece, regardless of how it was cut.
Mahogany is relatively dimensionally stable, so you probably don't have too much to worry about.
One word of caution: If you're routing the endgrain for any reason, be very, very careful and only take off tiny amounts of wood at a time. I had a huge tear-out problem on mine. I ended up having to cut a quarter inch off and reshape the bottom of the body. Next time I work with mahogany, I'll rough cut it on the band saw and then Robo-sand it.