New User Questions

The handbook will likely provide better answers than you will get here. So curb your impatience and start reading. https://musescore.org/en/handbook/4

You can get some short answers, but the manual is really the best place to go.

1. Select the note and press '4'
2. Select the note and press 'x'
3. Delete the system break, but if the staff is full, nothing will change
4. Create your own template with the guitar and choose 'custom templates' when starting a new score
5. A video? Surely somewhere, but I do not know. For chords with different note durations use additionally voices 2, 3 and 4
6. Choose acoustic guitar as instrument and you will get the sound of an acoustic guitar. Just changing the names doesn't help.

I've given some keywords you can use to search in the manual.

You wrote:
Is it possible to validate that all (or all in a selection) measures in a score match the time signature?

For non-metered measures (e.g., any not matching the time signature) see :
https://musescore.org/en/handbook/4/pickup-and-non-metered-measures
especially this part:
"If, at any time, the total duration of the notes and rests within the measure does not match the time signature, a small + or - sign will be shown above the measure."