It looks like the disparity on the UI resolution setting is that you were looking at it with the video still playing in the background. When you have refresh rate switching set to start/stop it sets the video mode when the video starts and it doesn’t change until playback stops. This includes when you return to the UI without stopping the video first. If you have refresh rate switching set to always then it would have switched regardless.
As Sam had stated you don’t actually want your UI set to anything other than 1080p. Refresh rate is personal preference. You can set it to match what you watch the most so there is less screen blackouts at the start of playback. If you want control over which video mode is used during playback the way to do this is with a whitelist. You can find a guide about that [here]
You mean sync playback to display? If so, do not enable that. It doesn’t work and even if it did it is not optimal.
As for the video switching while you looping the clip I would suggest that maybe there is a cache or buffer issue that comes up playing the same clip repeatedly. I remember doing a bunch of tests with jellyfish test files looking for more optimal Kodi cache settings. I ran into an issue playing the same files back to back as I would end up with non-repeatable results. This of course is just a guess and if you produced some debug logs more insight might be had.