I'm not lv70, but from my experience with DF trials and DF in general is if you've been running it as a healer for those 70 levels, you should be well adept at being adaptable by this point. You're not going to get the same healing partner from instance to instance so there really is no steadfast approach to a DF trial other than be ready for anything. The other thing you have to consider is to the other healer in your group, you ARE the OTHER healer to them, and they don't know what to expect from you.

DF trials tend to have lower clear rates when two or more people cannot perform their tasks for whatever reason. So when hiccups like over-healing happen, it really doesn't affect the ability to clear the instance despite low optimization between the two. But if both healers fail mechanics, then a clear is unlikely. I think this is the difference between savage clears and DF clears TBH. Because in DF, typically an experienced healer can still take care of everything should the other healer be inadequate. I don't do savage content, but I'd imagine both healers need to be on their game.

FWIW I do do EX trials with FC mates, and I guess it's pretty similar to savage. That said, the challenge I find here is different. It's more about coordination and memorization here. What's coming when, and who is doing what, and getting this down along with your part. In DF, there is no coordination. There is only adaptation. There are rare times when I have synergized with the other healer without words, but we're talking maybe 1 in 50 queues and that's on the optimistic end.