Hello JayWebb,
thank you for your answer. I understand that many of the issues I mentioned could be solved by making the poses exact (the artifacts and fingers leaning outwards a little) or by swapping the entire rig with EPIC’s rig. Unfortunately, the theory behind retargeting makes me wonder why such a huge visible glitch only appeared in one of the fingers - this seems like there is something wrong with it and I would like to find out what it is (and also why it only appeared in one of the animations).
In order to find the core of the problem I tried messing with bone orientations but I guess this is not the problem.