While I agree what she did sounds awful, I do think it could be a bit more complex. One thing is that she offered initially. You have also said you had no other options, so MIL may have felt she was leaving you no worse off than if she hadn't initially offered - sounds like you were intending to go alone until she offered, so she may have seen it as retracting an optional bonus, rather than taking away something vitally needed.
Also, if she didn't come because dh was planning to work, then that suggests she was rating her convenience over DH's work, not over your wellbeing. Dh planning to work doesn't make it sound like it would be terrible for you to be alone... it feels like if your dh understood how vital it was for you not to be alone, then he should have been willing to cancel work to make that happen - or go the extra mile to get her home some other way.
And we don't know how that conversation went down. Did your DH tell her how important it was and ask her to please make a special effort or did he get stripy and say -fine then - if it isn't convenient, don't bother - we'll manage"?
I completely understand how hurtful it feels, especially when your own mum wasn't alive to support you. I can only imagine how hard it was to go through alone.... I am so sorry you had to do that and I am so sorry for your losses.
But it also feels like if you knew how hard it would be (as you expect MIL to know) you could have arranged something else, rather than wait for an offer from someone who has a record of being unreliable? And/or your Hubble could have found a way to make it work.
OP, my understanding is that this happened three years ago, and you have never had a frank conversation with her about your feelings? I think for your own healing, it would help for you to at least express your hurt to her directly and, if you can, hear her response openly,,,,, (in an email even). If you decide never to speak to her again after that, then fine. But I would hope it would give you a greater sense of clarity and acceptance.
For your DH, I think he should be able to have a relationship with her but should not expect you to....