I think you are prob just anxious about leaving DS and you are focussing on how he's going to be looked after while you are away from him- completely natural. If it makes you feel better to give a some written guidelines, then do - hand it over with a sheepish smile if that makes it easier! I did that - it just made me feel better about leaving my DS, which I did find difficult. MIL understood - my own mum was fine too (they shared childcare), and it helped them to know that crying at 10o'c meant food, while a shriek at 11 meant sleep.
More important, general stuff - I just say. More friendly than writing it down, and focusses the mind better. So ask her to always make sure the harness is on - keep the peace by saying he tries to escape otherwise. It's difficult for them to go against that if you've specifically said. I say please don't let them in the kitchen while the hob is hot -that's my fear (I go cold typing this!), so they respect that, although I know that if I didn't say, they'd be fairly relaxed about him being in the kitchen.
I do speak from experience - before I left DS with MIL I made my husband arrange a time with his mum for me to go round and do a 'safety inspection' - what was I thinking!? Even I felt embarrased when I got there to inspect the video (finger insertion worries...) and and dole out plug guards! But they took it all well, thankfully!
But if it makes you feel better - say. He's your child, you all love him and you all want him to be safe and happy. Going back to work is hard, your MIL should realise that, and if she doesn't then tell her you are worried about it. She should understand. Good luck.