Welcome to the forum!
I can give some brief responses from my point of view:
1) It sounds like she wants to be in a relationship with him. So that is more on the polyamory side. Someone once said that monogamy is a subset of polyamory. In a sense, it is a label. If you are supporting ethical nonmonogamy and multiple loves, I think you could classify yourself as polyamorous if you want to. Though in practice, many people would consider you monogamous in a polyamorous relationship.
2) It sounds like you are dealing with NRE (New Relationship Energy). She just needs to be aware of this and try to tone it down a little around you. And you can realize this is a natural response to a new relationship, not a relfection on you.
3) I think JRiverMartin did a good response to this. The truth tends to have a way of getting out. I think TK should try to be honest with his wife so she can deal with it.
4) I think that once the kids are out of the house, that you should not have to pretend to be other than you are. You can just say she is with a friend. If they keep questioning, then let them know you are ok with what happens. But I think that since you know your kids better than I do, you will have to judge this.
5) I think as long as you are honest with each other, you will have many years of fun and frolic. It is hard to guess at long term consequences of your actions. Who knows? Maybe if you didn't persue it, you two would grow more distant?
6) I think menopause affects everyone differently. This may be something that a doctor will be better able to answer.