What is the simplest way to change the spacing between lines, both as the default spacing and when you need to alter the current spacing format for a document to a smaller or larger spacing? I’m not immediately seeing how, and I’m in dire need of knowing!
No point in writing nearly the same title in the msg area. Not writing such a long title would be preferable, but nothing major.
Open the sidebar, click on Styles&Formatting icon, right-click on default style, modify, Indents&Spacing tab, Line Spacing.
Good lord … there is a difference between a title and a question. These are 2 different things.
Please keep you title short (and readable) in the future 
This ain’t an answer.
See the Writer Manual for a full description of the many options available to format text.