Other mistake: Towards the end, Chewbacca and Beckett have walked some distance from the landed yacht. They come up a small slope and are faced with Han, waiting for them. But an aerial shot shows that slope is very short and just runs down to a cliff. There's no way they could have got to the base of the slope to walk up it in the first place.
Other mistake: Apparently, the route to Kessel is the same no matter from which direction or part of the galaxy someone needs to get there.
Chosen answer: Qi'ra's motivations are purposefully vague. It could be she killed Vos because she loved Han. It could be a simple power play, as she did not like being a slave to Vos. It could be a combination of both: she was able to save Han and also gain a position of power by killing Vos. We'll likely never know the answer as a follow up to Solo seems unlikely at this point.
BaconIsMyBFF