It's a tough spot for a H to be in for the very reasons you mentioned and due to the stigma attached to depression and especially PPD. Other illnesses people rally around you, support you, bring you food. Depression, people stay away, we hide it.
I also don't think that all women who come to this point of wanting something more/different are depressed.
But you W has that history and depression often recurs (or it waxes and wanes without treatment) and can be a chameleon.
It's useful to you because she's still someone you love (I think) if only as the mother of your children.
You can't make her seek treatment or even evaluation, and any suggestions from you at this point are going to be shot down.
Depression isn't a hall pass, there are too many very good ways to treat it out there. The diagnosis (and she hasn't been dx'd) also shouldn't be used to leverage custody.
It's just another possible piece to the puzzle.
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss