There was a fade to black between Dolores killing GTA5 and her stumbling upon William and Logan, the popular concenous is the fade to black is a indicator of the timeline/storytime changing from present to past, there was an example of this in the last episode too right after Maeve says "time to write my own story", it fades to black, then fades back in to Dolores and William approaching the burried town on horseback.
Later in that act she has a flashback to the town where the staff are training the androids to dance, but just before that, there is a very brief shot of her facing the town and it's completely empty, there is then a strange sepia type transition to a populated town, and eventually its revealed the town is burried and she has her mini meltdown about Arnold.
So its not safe to assume the flashback of MiB she had in the barn before escaping happened in the same timeframe as the following scene where she stumbled across W&L (remember that scene she flashed back to happened in episode one when MiB killed Teddy) conventional storytelling techniques suggests the two scenes are chronological, but this is a Nolan show and history tells us you cannot trust anybody in that family to tellba story in a linear fashion.
I suspect what is hapenning is Dolores is retracing her journey with William through reliving the memories, we occasionally see a shot of her alone, or a mirrored third person perspective of her looking at herself doing something, whatever journey she is on is in the same "time" as MiB and Teddy but it's being told to us from her perspective as reliving the memories of her time with William.
So I think William is MiB and eventually she will meet MiB in present time on his journey, William will initially be shown to be standing next to her, the internet will then explode with cries of "told you there was only one timeline", only for a few seconds later for it to be revealed, in the same manor the hidden door was to Bernard, that William was never with her at all and was just a memory, she will probably call out for William, only for MiB to answer "I'm here", the internet will then explode for a second time.
Remember both Theresa and the Delos board chick complained that Ford had recently dug up an old town on the fringes of the park, and Ford himself told Teddy there was some truth to his new Wyatt back story, I.e. Wyatt is based on Dolores or Arnold's actions which ultimately led to Arnolds death, which is the memory of the village being slaughtered the victims of it keep having, however every one of them has a different memory of who Wyatt is, likly to be revealed as Arnold to us eventually after they toy with us for a bit by showing it from several different perspectives.
Remember that Hector, Angela, Maeve, Armistice and Dolores are all present in the 'Androids do Strictly Come Dancing" group dance scene.