Converting from 30fps to 24fps

When converting 30 -> 24 fps you must decide whether you want to preserve the duration and skip (or blend) a frame here and there (and make the clip stutter or movements smear somewhat). Or whether you want to preserve every original frame as is, and make the duration longer (FCP automatic speed does this).

Automatic speed preserves every frame so it is smoother. But if there is audio it usually needs a setting to preserve pitch and even then it might sound unnatural. Also the movements might get unnaturally slow. It depends. In high quality footage skipped/duplicated frames are easier to spot. By default FCP skips/duplicates frames (25<->24fps conversion seems to be an exception where the duration is just slightly changed by default behind the scenes), frame blending or optical flow are options you might want to test as an alternative.

Convert all clips in Compressor to ProRess. Set 24 fps and use "Retiming quality" Best

The file isn’t converted; it’s processed into 24 in the project. The Info inspector always shows the original specs of the media.

No.

Very unlikely. You might be able to see it if you look frame by frame.

Use "optical flow" option on every clip