Tbh I'm not sure they spelled out the mcguffin but the daughter is in possession of something a lot of bad people want and the mc is there to protect and find it first...
I just went and re-read it, the daughter isn't in possession of anything as far as anyone knows.
The ex-wife said in the letter that the supposed "blackmailers" may believe she is, but ex-wife only said that she'd hidden an item that gay-husband had told her to hide, not that it was the information that the supposed blackmailers were looking for or that it was even hidden anywhere near the house much less in daughter's possession.
Ex-wife even said she had no idea what said information was.
I'll also say that we still don't even have any confirmation that there ever was any blackmail, blackmailers or even that ex-wife and gay-husband are dead.
All we have is an exploded boat, some DNA evidence that *suggests* that ex-wife and gay-husband were on the boat when it exploded, a letter that was conveniently prepared right before supposed death that could just be even more lies and an MC+daughter who have a family full of treacherous shits that can't ever be trusted again.
Oh also we have a lawyer who must have known that the entire family other than MC had remained in contact with ex-wife and daughter and as far as I recall didn't deem that information important to mention MC at any point, going so far as arranging travel for Deb behind MC's back.
So basically another snake that can't be trusted.
BTW MC's theory about Sanford being behind the attack can't be correct, it wouldn't make sense to target ex-wife and gay-husband in order to get back at MC for shooting Sanford's son, it would only make sense if MC was still with ex-wife and daughter.
Its a red herring.
Re-reading the early part of the story has reminded me of a few other things too;
Firstly, on the tarmac after MC boards the jet during the conversation between Bart and Deb, Deb asks if ex-wife had told him, he said she had, Deb says she would be annoyed if MC ever found out and Bart assured her that MC would never hear it from his lips.
Now its not explained at the time what they are talking about but it clearly can't relate to the previous topic they were talking about since in the scene before that Bart already lets them know that he is aware of MC and debs "special" relationship.
So in light of more recent revelations it really can only mean they were talking about how Deb has remained in contact with ex-wife and Milly and didn't want Bart, or anyone, telling MC.
Secondly, When MC discovers Astrid is on the plane she reveals that she specifically chosen for the job, the "American guy" that approached her had specifically said she was just the person he was looking for.
This indicates even more strongly that this was all being set up well in advance and makes it look even less likely ex-wife and gay-husband are dead, rather it looks like its some other elaborate plan at work.
I'm not even convinced Astrid ever worked for a commercial airline and just happened to live right next door.
She seems innocent when everything is taken at face value, but its all very convenient, she just happens to move in next door, instantly start hooking up, then at just the right time she suddenly gets a "job offer" and turns up working for ex-wife and gay-husband, she's immediately not only aware of MC and Deb's relationship but also totally fine with it, then she immediately needs rescuing once MC arrives in USA which gives a great reason for her to move in with MC and Milly after which she initially seems upset because MC was again having sex behind her back with someone else but very quickly calms down and decides she's fine with MC hooking up other people as long as he tells her.
After re-reading everything and paying closer attention I wouldn't be surprised if Astrid is a plant, working for ex-wife, as part of this elaborate death hoax.
Sure it could just be a coincidence, but really its all very convenient.
Edit;
This line from Deb is hyper-amusing in retrospect, knowing what we do now.
she spent years standing by and watching ... didn't seem to have so much of a problem with it when it was anyone but the MC doing the lying.
Such a 2-faced hypocritical bitch.