I know what you're going through. When I left DD for a trial session with Childminder for the first time I was a wreck, but that was a year ago now and I know DD gets alot out of the change of scene and doing the things she doesn't do with Mummy on the days I'm at work. It took a couple of months to get used to the set up and if I'm honest it took me longer than DD to settle.
As for baby sitters, go out local, food, cinema etc. short and sober so you can get back easily if you need to. Mummy and Daddy time is V important but you need to be able to relax into it.
Everyone is different, so don't feel you have to push yourself - mummy weaning takes time too!
As for MIL it's an offer of help and as long as you trust her to keep everything safe and phone you if anything goes wrong then take the help. Whatever your relationship with her she's still your ds granny, and grannies are important.
If your DS is a good sleeper I'd try the evenings out with your partner first. I found it easier to leave someone in charge of the TV remote whilst DD was tucked up in bed. If they wake up screaming then it's an obvious sign that the babysitter needs to phone you.
DD is 2 and I haven't left her over night yet, even though Grandma's are happy/desperate to have her sleep over.
It does get easier and it's lovely when you pick them up and they're so pleased to see you.