This might be a point where you need to take a stand. It's obvious that your idea of what "No" means is wildly different than what your husband's idea is. (You meant "NO", he meant "Okay, as long as you change the parameters by a minute amount").
And if you give in, you've taught your MIL that she can hijack a family reunion, and force you to attend a shower you firmly stated you didn't want, if she's persistent enough.
As others have said, this isn't unusual behaviour for someone who has grown up with a manipulator - he's been well trained to go along with what his mother wants - and breaking free is hard.
One option - he can go to the reunion, but you stay home. Make this clear with your MIL, and if someone else asks, be honest about why you're not their - you didn't want to risk a surprise shower. If your MIL persists, she will look like a fool for organizing a shower that the GOH refused to show up for.
But yeah, I think couple's counselling is a seriously good idea, for the health of your relationship as you go on. The earlier you establish good boundaries, the better and easier it will be. If you plan to have kids, this is doubly important, because my observation has been that manipulative/boundary pushing family members become much, much, much more of a problem after kids are on the scene.