1. I've seen like 50 movies since I saw Three Billboards, so my memory on specifics is fuzzy. Take everything with a grain of salt and know that my thoughts may change.
2. I wouldn't begrudge anyone who didn't like that this was McDonagh using a female character as ostensibly a punchline to reveal something about Hawkes and McDormand. I get that criticism. McDonagh's been criticized for this before: using underdeveloped women to reveal something about main men/women in the script. That's definitely a valid complaint. For better and worse, everything in the script focuses on the main couple of characters. Maybe it's because his plays tend to be about three-to-four people, so that's what he knows. Regardless, I hear that criticism, and even though I wasn't bothered by it, it's not something I find invalid in the slightest.
3. As someone whose mother has been friends with several women McDormand's age who were left for girls in their early 20s because the man just wanted someone young and pretty whom he could control, I didn't mind her inclusion, honestly. I remember that she's mostly just set up as a punching bag for Mildred, but IIRC, there's also a moment in their last scene together in which Mildred addresses her as a human being for the first time. I get why people don't like how she's written, but it didn't bother me when I saw the film.