I also feel that "with my own arms" is an odd-sounding phrase in English. First, "I held her IN my arms" is much more likely to be used. (Though one could argue that, if I were a policeman arresting her, I probably WOULD use "wirh" instead...) Second, since I wouldn't be holding her in anyone's arms but "my own," the use of "own" is kind of redundant.
The sentence "I held her with my own arms" sounds quite unnatural. In English, far more natural would be to say, "I held her in my arms." I know that the idea of this section is to use the instrumental in Czech, but the English sentence given here is too forced.
I need some context here about where this kind of sentence would be used in real life. Like I could see something like I held her in my arms being something someone somewhere would say. But this specific one. . .is there some kind of grammatical context that I ought to know about or was there something lost in translation here
The idea of holding something with one's arms is really only used in extreme cases. For example, if someone had no hands, they might hold something with their arms. In 99.9% of cases though, it's in someone's arms. It really would be a very good teaching point to point out that the Cz instrumental can be used with the English "in" in this case.