Hi L.,
A couple observations: it's very clear that you do respect your MIL and are grateful to her for sharing her space. That puts you ahead of the game, relationshipwise. You also are taking the responsibility for the decision of being there, and know that other options are available.
Here's the picture from what you have described: You moved in with a grandma who likes to spoil a bit, and who was probably already doing this before you moved in with her. She may feel like she needs to novelty of being the doting grandmother and not the job of coparenting all the time for this to work for her. (Am I correct about this? I could be wrong...)
If it were me, I would have any discussions about future family plans or parenting when you and your husband are in a separate space from your MIL. She may be 'butting in' because you are talking about it in front of her, the way many friends would if they were living in a roommate situation.
For the sake of your son, I'd also limit any parenting disagreement/discussions to times outside his presence, unless necessary. I'm of the belief that children don't need to hear our anxieties, concerns or anything but positive or neutral language about themselves, period. If you and your husband are talking and she comes in and starts to advise, let her know that you'll think about what she said, but will have to come to your own decisions about things regarding your son. It's difficult enough in most relationships to agree on parenting strategies and what's going on with your kiddo, and even harder when you are considering input from three people.
It also might be helpful, if you have a specific parenting problem to address, that all three of you read the same book about it. Or at least offer it to her, and then there might be common ground on her knowing where you are coming from.
Ultimately, it's your choice to stay or go. I didn't have my son until I was 36 and very well established in my life, and I've also watched a sister have a child at a younger age and live with my folks; this was interesting to me to see the coparenting dynamic between my sis and my folks. There were both good things (she had great support from them in terms of room and board, reduced rent, etc.) and hard things (differences in parenting styles and discipline, my folks tended to 'spoil' my nephew-- he's still the most doted on and gifted-up, that firstborn boy in two generations!). Tough call, but it sounds like you are a caring mom with a clear head on your shoulders. Good luck!