Hello Helbo
Welcome to the twilight world, we would all rather not be here but welcome anyway.
Although every individual & their journey through dementia is different, it sounds as though your MiL and my Mum are at a similar stage. Mum can still read things, but not relate them 'spatially' if that's the right word.
I used to cut out the large MONDAY, TUESDAY etc. from the TV guide pages & pin the appropriate one to the arm of her chair each day, but very soon she didn't 'see' them at all and they just became a source of irritation to ME when she sat there & asked me
mad: 20 times an hour!) "what day is it?" without looking at it. Sometimes you just have to recognise when something which DID help is no longer useful, & give up on it.
I deal with all mail & bills and have done so for some years, even before her official diagnosis of AD, but she still has flashes of independence &/or stubbornness when she wants to see (for instance) her bank statement, but then gets distressed because she can't make head nor tail of it any longer. I usually try to get to the post first to filter what she sees (I have EPA) as that results in less stress & anxiety all round.
I too used to use a large desk diary to try & keep Mum informed of forthcoming appointments etc., but like your MiL she got muddled. Now I write down only what will happen
today on a single large sheet of paper, and stick it on the door between her sitting room & the kitchen (most frequently used & easily seen). If I have to go out to somewhere that I can't take her with me, I leave a note saying when I shall be back (allow leeway for delays!) If the day is to be uneventful, there is still a note saying "No appointments or visitors today" so that there is always something in that place on the door; theory being she will not forget (yet) where to look for information if she needs to. Of course, this is easy for me to maintain as I have moved in with her, and can change it after she's gone to bed without an argument about it!
Yes, you DO undoubtedly have to be 'sneaky' to try & maintain a peaceful life. It's very similar to coping with the "terrible twos" period with a small child. Use distraction rather than argument, bribery rather than enforcement, whatever it takes within reason.
And yes, it IS a steep learning curve. More so, I would imagine, for a daughter-in-law than for a daughter. More power to your elbow.