24 fps is usually use to shoot cinematic videos. Because we can see the blur pan shot, making it real. Most of the time cinematic film makers export their final video in 24fps while YouTubers export them to 30fps.

Shooting at 60fps allows u to slower the video by 2.5X. For example a 1 second shot in 60fps video can be slowed into 2.5 seconds video.

The slow motion mode in iPhone usually allow u to shoot 120fps or 240fps if u are doing 1080p. But it depends on which model of iPhone u have. Could differ by the model.

Yes. If u intend to shoot slow motion, best to use this. But if u just wanna do a little slow like 2 times slow, use 60fps. Because shooting at high fps requires a lot of light to be present. Otherwise u will be sacrificing ISO and quality of the image. More noise will present in the footage if light is not sufficient in high fps shoot

Yes u can do that. Bear in mind, this is just the general rule. Creative wise u can always break the rules. Best film always break the rule. As long as you shoot with an intention, select your settings wisely, I believe your audience will understand your intention

All the best in your shoot