mog is hard to shop for!

First, your mom's dress is gorgeous! That's a tough one with your MOG.... The black dress with a train seems a bit over the top for any wedding other than a major black tie affair, but if she likes it I wouldn't say anything. She'll likely look at least a bit out of place, but that's on her.... The two moms will likely only be in a couple of pictures together, and in a group shot MOG's dress is just going to look long & black and will potentially blend into the men's suits around her. Your mom's dress isn't informal, so I'd just go with everyone's choices.

The only direction you should give your FMIL is to wear whatever makes her happy and comfortable. She does not need to match or coordinate with your mom at all. If she wants a long black dress that's fine. You should not be approving her clothing choice at all.

My FMIL purchased a navy blue cocktail dress and showed me after she bought it (not for approval but, just as a hey look what I got), my mom hasn't bought anything yet. They are adults though and I completely trusted them to figure this out on their own.

It's easy to get carried away when you have to make so many decisions when planning a wedding. This is not one of them.FMIL can wear whatever she feels good wearing.

MOG should wear the dress she wants to. There is absolutely no need to match or coordinate anything with the MOB.

The train is weird I think, too bridal. I gave my mom and fiance's mom a color palette to try to match, it's similar to what the wedding party is wearing. Beyond that, I don't care what it looks like.

I really don't understand why people care what their parents wear. My mom asked me if she should wear a certain color, I told her wear whatever you'd like. My FW's mom wore what she wanted. They both looked great. Don't stress yourself over this.