I've been working on one of my "novella" posts, which give my thoughts on answers to a bunch of questions. Not long, in-depth answers, but a condensed version of all of the possibilities I've thought of. In the meantime, I wanted to comment on some things that don't really fit in with the questions I'm working on.
I don't really think that the family's suspects are responsible for most of the things that happened. Obviously, he must have followed her at one point if she saw him, but it could have been for a completely different reason (not necessarily innocent, but different anyway.) Some of the things I think may have led to them becoming suspects are:
She and M didn't seem to like each other
He sounds like he may have had some attraction to M
M got a bad feeling about him when they met
They're young and seem like they may be a bit mischievous (or worse) at times
The connection with her family to the keys and the horse ranch.
The social media sites where they, and their friends, commented a lot about drinking/getting high
Living close by
He hunts and may have night scope (or goggles?)
I think someone else may have taken advantage of these things, and used them to put suspicion on the couple and off the actually 'stalker'. I was thinking about how the whole thing could have started, and I'm wondering if it may not have been innocent in the beginning. M had just gotten home shortly before the tapping on the window happened the first few times (she didn't tell them about it originally, but told them after a few times and that it was always within about 15 minutes of getting home.) It's possible that in the beginning it was just somebody tapping on the window to get her attention so she'd come to the window and talk.
During one of these times, the stalker may have 'accidentally' seen her undress, and developed a desire/need to see more. That would account for changing to bathroom windows, climbing higher to get a look in the uncovered top windows, etc. There was also the knowledge that the couple often had drugs and paraphernalia in the home, and she would probably refuse to allow LE in without a warrent if they went there to try to catch him after an incident. This would give the real stalker time to get away in a different direction undetected.
If it was a friend or friend of a friend, the person would most likely be well aware of when M was going home, and also (since they did seem to have a lot of mutual friends) would know when K wouldn't be around to be blamed for things.
This is already getting way too long, so I'll end now and maybe contin ue later. The main point I was going to eventually get to here is that everybody seems to think the stalker (if one existed, which I believe did) was either a male who had an attraction or anger toward her, or it was a female who was jealous/angry, or a combination. I was wondering if it could have been a female who had an attraction, and maybe hadn't "come out" yet in her teens or early 20s.