I got the entirely predictable "I was never going to go through with it, it was just the thrill of the chase"
So, WHY HAD HE ALREADY WARNED YOU, THAT HE WAS GOING AWAY SOON?
He is still lying.
He wouldn't have started the wheels turning in your head, about an upcoming night away, if he had no intention of going through with it. Lies, lies and more lies.
Ignore MIL. She wants this whole thing to go away. It would be an entirely different story, if her son confided, that you had been planning a night away in a hotel, with an Ex. You'd be every name under the sun. Guaranteed.
My Ex MIL blamed me for breaking up our family, because I had the audacity to leave her son, who'd cheated on me with at least 10 women. Apparently, what he was doing was "having a bit of fun" whereas I was the heathen who was "breaking up the family". 
Prior to this, she had been a normal and nice MIL. But, it's surprising how far some parents will go, to protect their own flesh and blood.
So, ignore MIL and other family members, who simply want to pass the monkey back to you (him). Take time away from him. He's not going to spontaneously combust, if he stays with PIL for a few weeks.
This time can be spent really, really thinking about what you want. To be honest, if you had the funds and the time, I'd be thinking of booking a last minute break away with the kids - 2 weeks away, without him, in an entirely different environment. 