If his 16 year old daughter didn’t get pregnant we’d still be perfect 
Do NOT count on this... If it wasn't his daughter getting knocked up, it would have been some other event that triggered... A Mid-Lifer is like a

looking for a place to blow up....
Likewise, the AD (Affair Down) is a SYMPTOM, not a cause,. If it wasn't THAT particular OW/OM, it would have been a different one.
However, it sounds like, at the moment, he has his cake and is getting to eat it as well... He is leaving the house after dinner and coming back in the morning so he is spending the night SOMEWHERE... You don't think he is doing the shag-nasty with anyone but the question remains "where is he spending his nights?" The "with whom" part is actually irrelevant as it is NOT with you and your daughter. But you are the one doing his laundry, making his lunch, making his dinner... Why? What benefit do you get from that?
He wants to live a single life of going out when he wants and coming home when he wants? Fine! He can do that but he is NOT living at "Hotel Mama" so the other stuff that comes as part and parcel of being a family (meals, laundry, etc.) is NOT part of the deal... He wants to live the single life,? Then he gets the FULL responsibility of a single life.
Yes, I know exactly what the first thought is... "But that will drive him farther away." He is ALREADY GONE! You are living with a Bug in an Edgar suit (Men in Black reference - you know.. this guy - )

or a Body Snatcher....
But right now, he has all the benefits and none of the responsibilities and responsibility and accountability are 2 things that the Mid-Lifer hates with a passion... They want to have their little fairy-talle world that is full of pink cotton candy clouds, puppy dogs and unicorns that run around and fart clouds of rainbow glitter... They are after their next shot of "happy" because they have NOTHING inside that can fill that empty aching void (that we knew nothing about but that we filled for them until we couldn't anymore... )
You daughter may only be 7 but kids are a lot more perceptive than we'd like to give them credit for. My son was 9 and my daughter 5 when their mom went off the rails and dived into the tunnel. D(now 10) didn't get the picture but S(now 14) most certainly did. The fact that xW is a wallowing low-energy Mid-Lifer has also played a positive role in that we are able to co-parent relatively well. Since I live about 15 minutes away from them, S14 is with me 50% of the time and when he is with me, he "commutes" from my place to school and back. D10 comes every second weekend and occasionally other times too... If he has "checked out" as you said, your D7 has most certainly noticed and I assume that SD16 has also felt it...
You are going to end up being the Rock in the storm for them, the stable parent, because MLCH certainly will NOT be - he isn't even capable of it at this point.... any more than he is capable of being a husband...
So, back to my original point.... If you are essentially doing the work of a single mom, why is MLCH being handled as a married H and Father? He wants the single life? He can have it but he does NOT get the fringe benefits of being in a family at the same time... He does NOT get to have all the fun and no responsibilities... He wants to dance, he has to pay the piper...