Episode 4 greymother frame, ¿Easter egg, developers mistake or an implication of something worse?

[Glacia]

Not sure if this have been posted before, but i realized that there is a greymother/lilith frame above blackrock office desk, close to astrid briefcase.

This frame it is not generic, you can not find more in any region (correct me if i'm wrong) . The frame features grey mother and lilith/martin barker.

I find it very curious how this is possible, because Will already find martin barker dead at the milton farm, and he was a farmer, not a blackrock employee.

At the very end of episode 2, mathis claims he killed an old woman, and will call him a bastard. However, in the first scene of episode 4, mathis confirm that the 'old woman' was a random survivor near the dam, and that he killed her so she would stop creaming.  

So the question is, ¿Why there is a frame of greymother in blackrock?  (If we assume that it is not a development mistake).

Also, i always found very weird the route that mathis made throughout the episodes 😵. The group was forced to leave the pleasant valley because 'a monster' was hunting them (Aka very angry blonde lady..), so they captured will, then go back again to PV and then blackrock?  ¿Why they go to mistery lake in first instance if the plan was release donner?.