Definitely yes
No.
Yes, you should.
Follow your gut
Yes
Confront the husband first, then talk to the wife if the husband doesn't stop.
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.'
No. Let sleeping dogs lie.
If you have absolute proof that your husband is cheating with this other woman then you should deal with your own problems with your husband first and yes, the other woman's husband should know. This is not an uncommon problem and most people (whether a husband cheats with another man's wife or a wife cheats with one of her friend's husbands) is very common and the victim of the cheating generally does tell the husband or wife that their spouse is cheating with their spouse.
Yes you should before it gets to late and it will be more hectic than it would be if you were to tell him now
You should talk to him politely and tell him. You can tell your whereabouts about which he has been unsure.
If it is yes most definately.