This last episode had some decent elements but was just written odd. The whole time remnant thing is overall way to confusing. Only way I could see that working is if his future self was the one playing jay and sacrificed himself, not the other way around where he has killed his past self. Barry also should have beat the brakes off of him at least for part of the episode. Zoom put him in a wheel chair, ain't to much real talking after that. They then could have used the "darkness" to explain the power up with him escaping and getting Wally. The whole everyone sitting in the lab together almost looked ridiculous. Could have had that scene without Zoom in there and had him show up with Wally and an exchange was made. Like I said the ideas were there, the execution was just way off.
As far as the darkness goes and other theories, I think Zoom is either possessed by a time wraith or turning into one. I also think E1 James Solomon is the one behind the mask. I think it was stated earlier in the season Zoom showed up in 1991 on E2. I'm thinking he kidnapped him and is making him suffer for what his E2 father did. When he gets out, somehow he gets the speed force and we end up with the older E2 Flash.