questions on a choral score

You wrote:
How do I delete the empty measure in the middle of the piece?

If you mean measure 21, simply click into the measure to select it, then press Ctrl+Delete.
See:
https://musescore.org/en/handbook/3/measure-operations#delete-single-me…

How do I add the second and third verses underneath the last system?

Something like this?
https://musescore.org/en/node/286258

How do I change SATB to SSAA?

Double click on the word 'Alto' and, in Part Properties, change the 'Long instrument name' from Alto to Soprano and the 'Short instrument name' from A. to S. Repeat for Tenor and Bass, but change those to Alto.
-OR-
In Part Properties, click on "Change Instrument' and, in the instruments dialog, expand the 'Vocals' dropdown and choose accordingly. The names will change when the new instrument is applied.
When using this method, beware of clef changes and notes marked out of range.
See:
https://musescore.org/en/handbook/3/note-input#coloring-of-notes-out-of…