I'm a lawyer by trade, so I am always wary of posting here. Please do not consider anything I say as legal advice. Only an attorney you hire or retain for yourself can give you legal advice.
My prior thoughts stand - i.e. why the quick turn around. As I see it, there are the following possibilities:
1.) Hellfire might be right; your WW could have been dumped and ran back home. She might have built up a sexual relationship to be something more and he dissuaded her of that notion at some point leading up to this. Remembering dating? How many on here got the talk starting with the phrase "where is this going?" Same idea. It could have happened that night or days prior, but the import of this could still be there.
2.) She weighed her options. She could stay with you, her children, her safe life... or she could jettison that all and run off with a man-child. Or strike out on her own free from everyone? What was she going to do? Move into his shared apartment like a college student? That choice seems obvious.
3.) She is committed to leaving, but did not like the lawyer's take on what would happen. States have different laws regarding your situation. Some, if a marriage ends due to a provable affair, might discount alimony. If your children are older then she could not count on child support either for a long time. Remember, your wife is pushing middle age, has just gotten a college degree and we are going through a pandemic and a likely recession. This would quite possibly be the absolute worst time to strike out on her own. Even if she was entitled to half your business that still might be a very sticky point depending on business valuation and income produced.
The second option is what you're focused on and hoping that it is the one. Notice, I did not put in that she suddenly found that she was in love with you. Maybe your wife was in love with you, and was for 19 years. But, for the past year, she has not been in love with you. (Some would debate this, but I don't think it's possible to be in love during an affair as a matter of logic.). A year is a long time. She might have been playing this out over months as the affair dwindled... wavering back and forth... and just needing that moment to snap out of it. I have no way of telling. I just consider it unlikely so I did not include it. I will reference it more below... it's possible but that sudden snap just does not seem likely. That shift either started months earlier and this was the climax, or it has not happened, as I see it.
The thing that sticks with me is the lawyer consultation (just because it's close to home). I see two reasons for the consultation happening:
1.) She could not live with the guilt of what she was doing and she planned on running away from it before you found out. This, in my mind is possible. Perhaps the shame started to get to her and she did not want to face it, so she decided to run and needed to know what it would look like legaly. Perhaps she was about to be outted or thought that she would be and decided to run before the guilt hit. Perhaps she convinced herself that she did not love you and that she had to get away, to alleviate the guilt. Long story short, the guilt may have been eating her and she wanted to get away. She needed to be prepared for that so she consulted an attorney. She would give you a nice settlement and start life over guilt free.
2.) She could have decided that she wanted out, absent the guilt of the whole affair being a factor. This would be the exit affair, where the overriding desire is to leave, not run from the guilt. Her man-child may not have factored in here. He could have been a distraction while she got out.
People end marriages for any number of reasons. But, in the case of affairs, I think that the person having the affair, and examining ending the marriage, is either running from something or running to something... but they're always running. They're not considering the totality of it all. I think your wife may have been running from the guilt.
What stopped the running?
The timeframe of the lawyer's consultation, and subsequent reversal, leads me to believe that it had something to do with not liking what she saw or how things would work. Call me cynical.
She might have been told what dual custody would look like for your younger child. This could be changing school districts after a home is sold, changing friendship groups, less ability to fund college... a whole host of negatives. And, if it's just a few years, maybe she could exist through it to get her kids over the hump. Then, maybe reexamine leaving in a few years when things calm down. And, then her leaving wouldn't be about her affair, but rather the extraneous marital issues independent of the affair. Believe me, some people have marriages that are marriages in name only. If this is the case, then it might have been easy for her to dump her boy toy. She changed from running to an independent life to running to save her kids.
Likewise, there are certain rules about alimony and certain ways that judges rule regarding alimony (if it is owed). A twenty year marriage might be a magic number in your jurisdiction. The judges that would examine your case might be very 'anti-affair' and penalize alimony as a result. Or, alimony in your jurisdiction might be drastically less than what she's read about online. She just got a degree that you gave her, maybe there's the idea of imputed income as a result. I can't give advice here.
Remember, she's fifteen years off retirement (maybe) and just finished her degree... and now a pandemic hit. A few years of child support and the possibility of decreased alimony might have scared her and she came home to fake it and figure things out.
Again, I have no idea.
If you get a lawyer's consult - and I STRONGLY encourage you to do so - you might consider asking what it would look like if your wife got the consult. Have them run the numbers. You know your wife... what would have snapped her out of it?
There's also the possibility that driving to her friends house she passed a restaurant you used to eat at, or a place you used to walk, or a pool where you took your kids and she had that moment of asking herself... "what am I doing?? I'm being an idiot." As I said above, this is unlikely, but possible.
Maybe I'm a cynic, but I can't get the lawyer's consultation out of my head. When I see this, I can't help but be cynical.
My advice is to consult an attorney. You don't have to do anything with the advice, but you should talk with one, even if you have to pay for the consultation. And, if there is nothing "time sensitive" you can just take your time, digest your emotions, and see what comes out of this. It might be great and it might be bad. Only time will tell. There might be something time sensitive here that she is waiting out, or there might not be... that's the reason for the consultation. You just need the information.