Fun puzzle. I wasn't thinking about the mystery until I got to the entry "Agatha" (I was about 3/4 done with the grid) and remembered there was a meta to solve and thought about her books and saw AND THEN THERE right and looked for the "were" and I hadn't enough of the puzzle to see the none off the bat.
So clever but was there any reason or sign post to look for the key words th..... Oh. Now I see it! Bee line, Bow Line, Fence Line, Bread line etc.... But I'm not familiar with the terms OPEN LINE, SNOW LINE (oh, wait... I've heard of snow line) or HOT LINE.