Experienced Auntie here of many decades.
I would not dream of keeping toddlers away from mum & dad for a moment longer than agreed. Of course, traffic jams, delays etc happen but whatever mum & dad say is what happens.
I am in ‘loco parentis’ & so responsible for their safety, wellbeing, ice-cream & having optimal fun. I have no wish or grounds to disregard their parents’s wishes & preferences.
It seems to me that your MiL did not try too hard (or at all) to make the train home. She knew the hosts well & could rely on them putting the brood up. Were the venue not known & friendly, she sure as hell would have made the last train out of Dodge City, because she could not rely on anyone & may not have enough money for taxi/hotel.
So MiL either planned this or let matters take the course because she knew she & the kids would be OK, but you & your wellbeing did not cross or figure in her mind.
Yes, you got a few hours to yourself - she could have gone to the park or had them at hers - BUT she also gave you a night on your own, early stages of pregnancy, missing your LO & not knowing how they were & in a place with people you did not know, plus she dismissed your concerns & blocked you getting your child. She caused you angst, loss of sleep & annoyance.
You gave her your trust & she abused it.
You can both come back from this.
I reckon your MiL is prolly very responsible & a good Gma, To give her the benefit of the doubt, it seems to me that she has assumed that what works for your SIL & her DC will also work for you but a)she has not taken into account that you are a more recent mother, b) you are vulnerable - OH away & early pregnancy hormones & c) she & SiL have had time to get to know each other & their arrangements have evolved.
Time to have an honest conversation with MiL.
Do not bottle this up.
Set some ground rules so that your benefits are mutual.
Pregnancy hormones speak to an evolutionary truth, do not diss them.