So sorry you have to deal with such a stressful situation, especially during pregnancy. It sounds like MIL has a control issue. At some point, your dh needs to stand up to her and let her know that it is HIS family, not hers. She shouldn't be calling the shots and expecting to be at your birth without consulting you about it first. I would go ahead and be upfront with her and let her know that you are planning a homebirth and only want your mws, ds, and dh there. OR depending on how old Orion is (lovely name, btw) you could assign her a job to keep him while you are in labor. Just let her know that it is an extremely intimate time and you will invite her over when everything is over. If she pouts and flips out, who cares, really? She's an adult and is obviously used to getting her way all the time. That is just not realistic. But on a totally different level, she does deserve your respect, and lying to her isn't the way to do that. In fact, if you do decide to play that game, you're being just as passive aggressive as she is. You know? It is a crap situation any way you look at it. But remember that it is HER problem, not yours. If she tries to come over during your labor, dh is responsible to get her out. She may be upset, but she'll come around when she's ready to see her new grandbaby.
In my own experience, my next baby is due in November and MIL has already taken off time from work to come down the first week in December. My original due date was 11/27. I explained to her that the baby might not even be here yet and to plan to come down sometime after the new year, WITHOUT her dog or her other grandkids. She is mad, but she'll get over it. She's going to be even madder when we tell her that the new due date is 11/14, and we still don't want her and FIL to come down until after the new year!
With dd, she and FIL had planned to come down when dd was 2 weeks old. I asked dh to call and have them postpone their trip by 2 months, which they did. It was much better that way.
Good luck! Either way you choose it's going to be tough. I'm just all about being as honest as possible. That way, it's easier to pick up the pieces afterwards.