Stems on "small" (inspector) are on wrong side.

What must have happened is that you used your mouse wheel to inadvertently scroll through the mirror head options without realizing it. Inadvertent scrolls are possible too often in MuseScore and is one thing that would be nice to have fixed. Most dropdown menus in Windows don't allow for unintentional scrolls. They require that you click them before changing them.

It's possible as mentioned your mouse wheel accidentally triggered the mirror property. it's also possible you inadvertently press the keyboard shortcut Shift+X which does the same.

You can modify properties for as many notes at once as you want. If you have a range selected, you can use the Selection Filter to limit the selection by voice if desired, then press the Notes button in the Inspector to limit selection to just the notes, thus enabling the note-specific controls.

Normally you'd want to use the "Chord" section of the Inspector to makes notes small. that way it gets the stems and other markings as well (accidentals, articulations, etc). The "Note" section affects just the notehead and markings attached to that note only (eg, accidentals). Only time you'd normally want that is if you have a chord of several notes on one stem and you want just one of those notes within the chord to be small.