All sounds a but Jerry Springer tbh.
MIL must be a regular drama lama to be raising it on speakerphone with kids in the room.
Understand that you would have chosen at 17 not to get involved if a 9 year old stepchild was going to be a weekly feature, OP, but that's hardly where you're at now is it?
Give it a few days to settle. Try having a conversation where everybody is dealing with the facts as adults.
Is it really going to have that much impact on your lives now if a 32 year old occasionally wants to see DP?
I'd be surprised if he wasn't prepared to share results of DNA test if he didn't feel quite so flipping boxed into it - between his mother's unhelpful announcement & your (understandable) kneejerk demands.
If you're just going to up sticks & leave for this alone OP, like others, I think there must be a LOT of other problems & resentment bubbling away already.
Take some time to let things cool & see if you can find an evening without input from any other relatives around to just go through the facts, what facts still remain to be cleared up, and what actual practical impact this might have on your lives - I suspect it's a whole bunch less than you might be thinking?