This first one doesn't bother me too much, given all of the suspension of disbelief needed in these stories, I find this quite acceptable. However, the I Ching presumably isn't meant as a means of time travel. When Barnabas used it, he thought he would be contacting Quentin's spirit. He didn't know he would travel in time.
Even if the hexagram of change does mean a change of time, it wouldn't necessarily mean the same time. Yet Barnabas and Julia both go back to 1897. Since they went "back" in time, why does Petofi assume he'll go forward? And why necessarily to 1969? Yet both Charles in his attempt and then Petofi do, indeed, go to 1969.
-----
The more problematic one is Petofi's hand. I think they forgot what they wrote about it. The entire beginning of that story was "our" people's possession of the hand, Petofi's search for it, and then finally getting it reattached. He went 100 years without it, and if he didn't get it back at that point would have died (presumably).
Now, I'm sure he still had formidable powers without it, but why are Kitty and Charles so afraid when he holds out the hand and asks if they remember what it can do. They could never have known the Hand.