Re: mil
Been there done that. (as in my mom did that to me) if u need details on my thesis report it’s in one of the messy bathroom threads.
But yes op some ppl actually have a real aversion to it, she might not be doing it just to mess with u. My mom would get pissed when we had guests and they’d leave hair on the bathroom floor. So when we were going to have guests over, she removed combs and brushes from all public areas (figuring the less they saw a comb the less they’d want to comb it everywhere). She’d also lightly hint about what a mess our plumbing was and how we have to be so careful with our hair here (in the middle east). In Pakistan ppl have those humongous floor drains and no mesh catcher so everything just plops through. They dont care if they’re going to block some major pipe in their house and the system backs up. Theyre just going to have the kooray wala (trash collector or someone along those lines) come and declog their main gutter. That is also why some ppl in pak don’t care how much gunk theyre passing in their kitchen sink (as long as they don’t have to deal with the consequences the problem does not exist).
Apart from leaving ur hair down, do u also randomly keep running ur fingers thru ur hair or keep handling it, shifting ur partition here and there. If u do that too, that’s causes quite a lot of hair fall and that too all over the house. I could see my mom getting antsy when she’d see ppl do that in our house and would really have to control herself not to say anything. (if they were not live in guests then she would never say anything, anyone staying over 2 weeks could expect dropped hints if they were the hands thru hair category). Btw hands thru hair ppl, I’ve seen them sit in other ppls houses as well as their own, and run fingers and then any lose hair caught between their fingers, they get rid of it on the floor, now that is just gross, that’s what trash bins are for. Ppl who never really thought of it as a mess on the floor are pretty absent minded about this habit, but for someone with hair aversion, it is the equivalent of chewing with ur mouth open, noisy eaters, nail clippings flying here and there, or add any of the other regular irritating stuff here.
Im sorry my mom has morphed me into herself (somewhat) and yes it is for that reason that I always keep my hair tied (and now find it uncomfortable to leave them open) and also pick up any stray hairs off the floor if I ever spot them. U know my mom would even get irritated when I had just showered and was only leaving them open long enough for them to dry a bit! Yes I have been traumatized by my childhood and now watch other ppl run fingers through their hair and spot a pile of hair next to where they’re sitting, while they obliviously continue with their group discussions. 
Hope this gives u some insight on ur mil’s behaviour. I bet if u observe her daughter she’s particular about it too. (unless she was the rebellious kind and decided to not give a rats a$$)