Maternityleave,
You were able to see your ILs under a different light because:
1. You just had a 2 week break from them
2. They were nice to you: cooked dinner, did not guilt you or complained for the 2 week break
3. They decided to behave like human beings for once
Are your ILs always like that? From your posts, this is the exception, not the rule. How much longer do you think this honeymoon period will last?
Feeding your baby the bottle and ignoring your rules for nap is a huge red flag and boundary stomp. Please do not gaslight youself.
Your SIL just told you that she wants to play mommy to your baby and she doesn't care about your parenting rules. Was she even called out or reprimanded?
You and baby suffered tremendously from this. Grumpy baby, disruption of the sleep schedule, etc. Did your dh try to help you during this time and saw the impact on his son?
By allowing one day a week with them, SIL has your green light to do what she wants. Guess what? You will be doing the work to get your baby back in schedule (resentment anybody?)
Giving them rules to follow and trying a setup to facilitate following the rules won't work and you know it. They will still do what they want.
Are you doing this for MIL or SIL? SIL seems to be the most benefited from this deal, not mil.
If and only if ILs can behave consistently like this last visit with you for a long period of time AND they respect your parenting riles, then maybe a day a week is a compromise.
What are the chances that is going to happen?
At this point, you become an active participant in this drama and then it is a you problem, not ILs or dh
Think of an alternative plan when this 1 day a week fails. What are you planning to do then?