Ah, it sounds like MD is the one who's reluctant to have the "relationship conversation." I wouldn't give up trying to talk to her about it, just give her a nudge now and then.
As for the town you live in, it sounds like you have three choices:
- keep your poly relationship a secret,
- break up with MD,
- move out of that town.
I personally think you have a good relationship with MD so I wouldn't vote for that option. I'd vote for the other two options. There's no rush on anything, so for now you can just keep your poly relationship a secret while you figure out what to do.
You won't have enough info to know what your next move is until MD opens up a little (e.g. elaborates what she means about the "too bad about this town" saying). And you can't force MD to open up, so it looks like you're stuck giving her a verbal nudge from time to time. Perhaps you can press a little deeper as time goes on, but do so carefully, checking MD's "emotional temperature" to make sure it's not freaking her out too much.
I know it doesn't seem this way, but you're very lucky in that you have a lot of love between three people and a husband that supports you in this. Sure there are some challenges, but treasure the good, and just tackle the challenges a little at a time.
Keep us posted on how things are going.