I think first of all you need to have a very serious talk with your dp about how you both see your relationship with this woman panning out in the future.
If he refuses to stand up to her, then that isn't going to change, and you might want to consider if that's the sort of family dynamic that you want to marry into.
Personally I always struggle to understand how people could get married without their parents there even as witnesses, or not involving them in any way shape or form. I know everyone has their particular circumstances, and some people genuinely are horrible, and sometimes it's a gamble people want to take to avoid stress on the day.
But a wedding is hopefully a once in a lifetime event. To cut a parent out of that can't be undone. Personally I would find it unforgivable.
But that doesn't mean you just have to roll over and do whatever she wants you to do. Why don't you try and find a compromise with your dp and then tell your mil what is happening.
For example:
You two get married wherever you want, in whatever circumstances, with both sets of your parents as witnesses. You then both go off together, and your parents get lunch somewhere to celebrate. That way you don't have to hang around, or invite any guests, but they get to actually be there and to celebrate it a little afterwards (amongst themselves).
If you do this I'd recommend booking the smallest room available that only holds 6 people or something, so no-one else can be invited.
Alternatively, you two could go and do your own things with random witnesses, but arrange to go out for a dinner with immediate family and close friends a couple of weeks after the wedding. It will cost MUCH less than having an actual wedding reception, but both sets of your parents get to feel like they didn't completely miss out on the fact that you two have made this huge commitment to each other. You don't have to sit near your in laws during the dinner, which might make it more tolerable for you, and you could always try and sit her next to someone equally awful, or someone who doesn't mince their words!