God, this thread is weird. It's like people are answering different posts from the OP to those that are actually there.
OP offered MIL involvement at the start. MIL declined. OP is not really a frilly wedding type of person so is accepting all help offered from friends and family, and as her parents are paying for most of the costs, and they are local, they're naturally involved.
MIL's contribution has been resentment that the groom wants his own brother there, because brother isn't also her child; anger that the bride and her mum didn't schlepp an 8 hour round trip to hers to pick out wedding dresses so she could be included because it was clearly too much to expect her to travel to them, and upset that the ceremony will be one that suits the engaged couple, rather than MIL's own religious beliefs. Oh, and she wants her friends asked to this wedding she's not paying towards, rather than immediate relatives of the groom. And people are huffing at the TOTAL unreasonableness of the bride in not eagerly falling in with all of these plans? Okay then.
OP, you say she's normally lovely and you get on well. I think the idea of playing to her strengths with a secret project is a good one. Could you maybe ask, if the cake is fruit, if she can make two bottom tier size ones, so you can wrap it up and store it ready for the first baby's baptism (if you want to go that road, obviously!) so they'll be made at the same time, and be more meaningful? They last better if not iced so the top tier tradition never made much sense to me. What could she contribute that might make her feel more involved - and is she on email? Could you take lots of camera phone images to send her on a regular basis, so she knows what's happening?
I suspect her approach is because this is a bittersweet moment in some ways. Her baby is truly grown up, and will start his own life as a fully independent adult in a way you can't ignore. It's a rite of passage that probably reminds you of your own mortality - having babies did me, and I imagine their leaving school, uni, then marrying probably does as well. She wants to be a part of it and while she's behaving a little selfishly in how she expresses that, if you can find ways to offer involvement she may feel less compelled to try to grab at it, if that makes sense?
Weddings are like holidays: they can bring out the worst in all involved. As you sound wonderfully non-Bridezilla, I'd focus on keeping good relationships when all this is over, by almost all means necessary and possible. And be grateful she doesn't live locally. ;)