I think it was cut, and that the lack of blood on both parts of the clasp suggests this happened before any wounds were made.
Your research is interesting, but I'm not sure it has led to an accurate conclusion insofar as what happened to Meredith's bra. The cups of the bra are sprinkled with fine blood drops, aspirated as she struggled to breathe with that terrible wound in her neck. The doors of her wardrobe have the same fine droplets, albeit they are elongated because they were cast onto a vertical surface.
Meredith's bare breasts also revealed this pattern of blood droplets. And the side of the wardrobe, adjacent to where her head lay when her body was found, is likewise sprayed with aspirated droplets.
After struggling with her assailant in the area between her desk and the window, she fell to her knees in front of the wardrobe, and it was there that the assailant inflicted the fatal wound to her neck, which immediately caused copious bleeding on the floor. The bloodstains show that she was on her hands and knees for a few moments, struggling to crawl. Her left hand was covered with blood when she was found, presumably because she held it to her throat at this time while propping herself up on her right hand.
When she collapsed, the killer began the activity that culminated in some kind of sexual assault. He dragged her, face down, using her blue fleece jacket as a handle. It started to come off, so he removed it completely and dropped it where it was found, in front of the wardrobe. He dragged her until her head was on the left side of the wardrobe closet. It was then that he removed her bra and rolled her onto her back. She was still gasping for breath, but she was no longer capable of resisting the assault.
As to the question of whether the backstrap of the bra was cut or ripped apart, I could go either way, although I tend to believe Hendry because his analysis is so grounded in observable details. I do believe strongly that the disrobing of the victim and the sexual assault only began after she was incapacitated from the knife wounds. It is the only scenario that fits the overall pattern of evidence.
It would be interesting to use a knife to cut through a piece of fabric similar to the bra fastener, and then compare the appearance of the cut edge to the high-res photos of the bra fastener.