I agree with the sentiments re groove vs rabbet (having had to replace glass in a groove); however Sommerfeld Tools sells a glass retainer strip that...
I was looking for information of PEX failure modes and didn't really find anything. I did find what you say about sunlight. Memory says PEX shouldn't...
I'm with Mel, start cutting it off an inch at a time and you will get a better idea of what you are dealing with. If the checks go competely through...