One of the things Dominikk is also doing really well is that he is posting video and asking questions based on what he is actually doing. So, pointing other people to the way people answered questions for Dominikk, that were in response to specific questions Dominikk asked within the context of specifically what Dominikk showed in his videos of himself.....well it may not always apply to other people. I work with people on body mechanics in a field not at all related to table tennis and if 10 people asked me the same question about the same movement, I would likely answer all 10 of them differently based on observing what they were actually doing and what information they needed to answer the issue they were faced with.
Part of why the question of how far the elbow should be away from the body is not an easy question to anser is that it has to do with how far away from you, you take the ball. Within the restraint of (almost) the length of your arm, the farther away from your body you take the ball the more torque you racket will get from the stroke. And when you misplace your feet so you are a little further away than usual, or a little closer than usual, the distance of the elbow to the body will adjust. So, theoretically, if the ball is way closer than normal, you could have the upper arm jammed up against your body (elbow as well) and still take a good stroke with only your forearm. And if the ball is a little further away from you than normal, the elbow will adjust and be a little further away and it won't really change the basic mechanics of the stroke. Said differently there are A LOT of variables that would determine the distance of the elbow to the body from shot to shot. And then there are also variables that would cause the base distance of elbow to body to be different from person to person.
An example, Ma Long takes the ball a little further away from his body, as the base technique of his stroke than Dima does (and further than most Pro TT players) and that is at least part of why, in that photo above, ML's elbow is a little less bent than Dima's is in the photos of Dima. But neither is right or wrong.
But, I will say, if Dominikk is doing shadow strokes, and his elbow is rolling on a towel or on his hand and the towel or his hand is flat against his side, the elbow is probably at least 6-8 inches (15-20cm) too close to his body for developing the base technique he would like to develop.
If he was using his hand, it would be good if he was spreading the thumb and index finger away from each other and resting the thumb against his side and the index finger on the upper arm just above the elbow. But it would also be good if he could go back and forth from having the left hand there on his side and not having it there without the basic movement changing.