I think most stories really need more than one voice. Even if Towner was not an unreliable narrator, still there are things that happen in a story any narrator would not be privy too and so you need the others voices to tell you. I read one great book that was done in a journal style, of letters. You would think that would be all one voice and then you would either have to accept the writer as reliable or question it all. But even in this story, you needed to know someone else's (in particular) side of this story to get the full texture of it. Interestingly, since the book had no chapters, and even tho you would know the other voice when you came to it because it was not a letter and it was a female, the author and publishers put a symbol at each part that belonged to each rather than a chapter number. It was about the Orient in the 1500s, so one symbol, I think it was a flower was her voice, the other, an Asian symbol, was his. I really liked that not because I needed it to deliniate who was talking but because in going back to discuss it, I could easily find which chapters were hers to look through and which were his. Nice little trick for this particular book. But the main thing here was, the narrator couldn't tell all of the story you needed to know. You had to have her voice too. So yes, I think the same is true of the Lace Reader. There is too much going on to only have Towner's POV and because we believe she is an unreliable narrator, we also need those other POV's, to really find this out for sure.
Time shifts can drive me crazy. It depends on how they are handled. I am reading one book right now that part of it takes place in the present and part takes place in the past and just when you really get going good in one time, bam, the next chapter is the other! Then you spend a few pages trying to orient yourself to what happened there last and deciding to go with this story again and then it gets really good and bam! There we go again and time to figure out what was happening when you were in this time period last because they really seem like two different stories and it would be like going to a movie theater, watching a full 15 to 20 minutes of one movie, then dashing across the hall and watching 15 minutes of it then dashing back to the first one and trying to keep the stories straight. I think for me, it depends on how often they happen (how many flashbacks in a movie can you take before you want to walk) and why they happen. I think you have a good sense of where to put them Brunonia. For things to come together in the end, when you have an unreliable narrator, you have to have someone else trigger the events of the past that let to this point (this possible still point) by giving you the facts or their point of view, or her going back and forth in time as things are triggered to find the truth, so I think you did a good job there too.
On these two questions you ask here, POV and time shifts, I give you thumbs up on both. 
