I am getting the same spots on my mare, although I don't think she is narrow. She has a bit of a swayback though, so stuff bridges. Glad to hear the Wintec worked-well, not glad in that way-bc i had considered trying that too, but I think treeless will be my next step instead. I just got an Aussie from Down Under that they made according to my wither tracing. It seems to fit great (my current one is too tight i shoulders) until I am on it, then the saddle sits too far down and the shoulders are tight again. I have to send pics and saddle back and they can adjust it. If it still is a problem it goes back and I try a treeless.
Anyway, maybe try an Aussie with the serge panels, they tend to have less issues with bridging bc the panels can be awled to fit your horses back better. If an Aussie is an option or something you would like. I finally bought a bareback pad so I have something that doesn't torment her.