My heart aches for you, SJP! If it's any comfort, I've heard of other cases where a parent remained angry at a GP for a long time cuz of what was said to SS. Hopefully, DIL will get past this, eventually, and realize that you were just concerned for the kids. However, sorry to say, it may take a long time. Unfortunately, the other GM may be uncomfortable with you cuz of this touchy situation.
This is most likely why DIL is afraid you'll say something "derogatory" about her to your GC. Maybe it would help if you said something nice about her to them, now and then, in her earshot? But seeing how she "mistrusts any kind gesture" from you (very lovely of you, BTW), I don't know.
Anyhow, I'm sorry your visits were cut down. However, considering the long trip, you're probably better off going 3 X a year, instead of 6 (Yikes!). The short duration of the visit after that long trip doesn't seem fair, but it sounds like you make the best of it. Perhaps planning something else for yourself before or after you see the kids, like eating at a lovely local restaurant or staying overnight at a nice motel would make the trip easier and more pleasant for you?
I'm also sorry DIL won't allow any skyping or telephone calls. She is probably still too angry and distrustful for that. (Again, finding some nice things to say about her might alleviate those fears in time.) Since the GC are only 5, 7 & 8, I would keep the letters short, so they're more likely to read them on the chance that they actually get them. And I would maybe keep copies, so I could show them in later years and let them know I tried to keep in touch. As they get older, they might reach out to you themselves.
Did you realize what the consequences might be for you when you spoke to SS that time? If so, it was very brave of you to speak up, anyway. Kudos for putting the children's interests ahead of your own! I just hope the situation gets better over time.