Your husband should not be anything to his ex girlfriend and she should not be in your lives. It's time to sit down calmly and communicate with your husband that this is unacceptable behavior. If he argues you the point then tell him to make a choice 'you or her.'
Definitely yes
Yes.
Yes, you should.
Just tell them how you feel and hopefully they will understand.
plainly there is no way to tell him but just to tell him.
sure if your wanting a divorce, that's like leading a cat to a fish fry
That is a decision you are going to have to make. Understand that it may destroy the relationship with your lover. And you better be sure that it is an affair and not something that you are creating in your mind. Better to talk with your lover rather than send accusations out.
Follow your gut
Yes
Confront the husband first, then talk to the wife if the husband doesn't stop.
No. Let sleeping dogs lie.