In our universe there are several possibilities to slow the passage of time compared to an observer on earth. Namely travelling at a high percentage of the speed of light or existing in a high gravity field (close to the event horizon of a black hole) but there is no practical way to travel back and forwards in time

In your SF universe? Well, it is what ever you decide.

Tl;dr

Forward? Yes

Backward? Maybe (but probably not)

thinks about black holes as time portals

I'm no physicist, but this is such massive horseshit:

One of the peculiar observations that has emerged from the study of the quantum realm is non-locality. A change in a particle's state in one location can instantaneously influence another "entangled" particle somewhere else [...]

"A lot of physicists are very unhappy with the possibility of non-locality," says Adlam. That's because, for the effect to be instantaneous, the information must be conveyed from place to place at faster than the speed of light. This is supposed to be impossible.

Imagine, you've got a printed photograph. You rip it in half and put each half into identical envelopes. You mix them up and send one of them to Australia.
Now you open the envelope you kept and see that it's the left half. Boom! Instantaneous knowledge that the envelope in Australia contains the right half. Faster than light!

As far as my understanding goes, this is all there is to quantum entanglement. But because it's quantum, everyone loses their minds over it.