I've watched this thread from the beginning and I've read and re-read the OP several times, as well as reading all the replies.
First, and of course this goes without saying, is that like everyone, I'm so sorry for the two losses the OP is having to deal with. No parent should ever outlive their child and her grief must seem overwhelming.
I did, like others, wonder if perhaps her grief was overshadowing her in a way that skewed her rational thinking in some way, as I was surprised that anyone could be so, well....tone deaf to the whole situation.
The OP says that she took time off work and drove down but on opening the door her DIL told her she was "out of line". This is all quite telling.
Did she actually tell her DIL she was driving the 7 hours? Because if she did then it was surely at that time that her DIL would have expressed how she felt, rather than allowing her MiL to drive 7 hours and then tell her?
It also tells us that the OP has an income of her own, can drive, and that the income is enough to support the costs of running a car. She also is confident enough to do a 7 hour drive, and then the return trip.
I do wonder if maybe there is more of a "back story" here.
Sadly, the OP comes over as needy and controlling, and possibly jealous of the girl who took her son away, along with her "more well-off" family. If this is, and has always been the case, it may be the the DIL tolerated her MIL for the sake of her husband and son, but now sees this tragic situation as her way out of a toxic relationship? It would be helpful to know if there was any tension in the relationship prior to their mutual loss.
The good thing about taking to a forum for advice is that you usually get it, but it's generally honest as we don't know each other and we are all several steps back from the centre of the problem. The bad thing is that you very often get told what you don't want to hear.
Some people post their problems on a forum to try and get their own solution, or feelings, validated by others. We can see by the strength of the reactions on here that the OP is never going to get close to anyone agreeing with her actions and must accept that she has crossed a line that, unless she understands that her own behaviour has to drastically change, she may never be able to cross back over again.
I truly do wish her well and hope she takes some of the wise advise given.