PB, there will be no ambiguity when you move into piecing. Piecing is not something you think you are in. It is not something you might be in. When you are in piecing, you will know.
When a WAS truly wants to work on the marriage, they usually do not articulate that. It is very rare for a WAS to say "Ok, I am ready to start working on the marriage." And even if they do you have to take it with a huge grain of salt. Especially if their actions do not back up their words. And in this case it sounds like you are not seeing that in her actions.
If you really want to test this, then start putting your requirements in front of her. You said R2C mentioned a contract for reconciling. It is less a contract than it is a list of requirements that you will require to consider trying to save the marriage. Things like full transparency from her. She doesn't get to have anymore secrets. You get to know where and what she is doing at all times. Including GPS tracking app on her phone so that her whereabouts are known (and yours are too) at all times. You get access to all email, social mediate and messaging accounts. Etc. Then you also require she get into IC, you continue IC (please tell me you are in IC!), and MC for both of you to attend.
You'll learn very quickly how serious she is about "wanting to work on the marriage".
As far as why she would say "I want to work on the marriage." I think boat14 is probably pretty close: to buy time. A monkey rarely jumps from the branch it is on until it has identified another branch that will support its weight. Often times when a cheater's AP breaks it off, the cheater will go back to the LBS. That is what PLAN B is! Plan A didn't work out, so I will fall back to plan B. Likely she is miserable because she had plan A built up in her mind that it was going to be rainbows and unicorns, and now, for the time-being, she has resigned herself to the drudgery of Plan B. Plan B is what she had before, that she was unhappy with and that opened her up to another plan to begin with. If you go back to business as usual, I can almost guarantee that you will be setting yourself for a future BD #2. Many of us have been there.
I agree with boat. Keep DBing. Focus on yourself, be the best dad you can be, GAL, continue to work on your own self-improvements, and then do not stop working towards proper detachment. In fact, google: "self-differentiation in marriage". That is your new normal. That is what you want to strive for. To be an individual that has no codependency on another human-being, and is just as happy by himself in his own skin as he is as part of a committed, mutually-satisfying relationship.
One last thing, I do see the regular sex as a good sign. One of the first signs my wife was coming back to the marriage was regular sex. However, it is a bad sign that she is complaining about it. It, again, sounds like Plan B.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018