I think you just need to relax a little bit. It is natural to be worried about how you and your son will cope, as up until now you have only had him to worry about, but your MIL is right that you will have no choice but to get on with it - worrying about it in the meantime is only going to be counter-productive and make you more stressed. Your husband is also right in that if your MIL does things differently for a day it won't really matter.
I was very much like this before I had my second DS, and I would really urge you to get to grips with it now, before you have your second child. In my case, I didn't (despite leaving my first son with my mum who had had him plenty of times before) and to be honest I found it incredibly difficult to bond with my second son. Even after we came home from the hospital I was more concerned with how my first son was and most of my focus was still on him.
I went throught the motions with DS2, feeding him, changing him - but in all honestly, if the milkman had offered to take him off my hands I'd have said yes! I had no clue what to do with him, as "he didn't do it like DS1, DS1 used to do this..."
It took six long months before I suddenly woke up one morning, looked at DS2 and thought "bloody hell, I really love you!"
I think if I'd been more focussed on the second baby before he actually arrived things would have been very different. We now have two gorgeous boys of 9 & 7 who I love totally, but in different ways, and neither of them appeared to be scarred by having to put up with me for those 6 months!
Try to relax and spend a bit of time thinking about this beautiful new baby you're about to be introduced to, and who will be a blessing to all of you, including your son. x