I understand your worries regarding sids, I was extremely worried about this with my first ( and still am to a degree with my 2nd). As much as I think it’s completely normal for you and your husband to sit in the dark bedroom, I personally wouldn’t ask anyone else to do that. If you’re worried she’s not going to do it, I would either just set up the baby monitor and let her go downstairs anyway (better than her secretly going downstairs without the monitor) or I just wouldn’t go if it’s going to ruin your night 🩷
Completely understand your worry about leaving your little one sleeping in a room alone. However I agree with what the others have said. Could you even use the camera even tomorrow night, that might make you feel better about MIL doing it on Friday. I was very anxious about leaving my son in the bedroom alone, but this week we started using the camera and sitting downstairs for an hour or two before going to bed, now I've done it a couple of times my anxiety has definitely settled. Hopefully you get to enjoy your night on Friday!
I totally get the being in same room anxiety. We’ve had the same thing and when my mum/MIL has minded our little one I’ve just set up a Moses basket/bed situation in our living room so that baby sleeps there until I get home and then we take him up with us to bed. Honestly it’s so much easier if you have the option of them sleeping in a Moses basket, bassinet or even pram in the living room until you’re home and that way you don’t have to worry because them being upstairs on their own is no longer an option. X
To be honest I think it's a little silly but I do understand your worries. I do think it's unfair to ask mil to stay in a dark room until you come home. I think it would be a good idea to set the monitor up for your mail to use and when you get home and see your baby is completely safe it may make your anxiety a little better.