I found a way to set line spacing without using the Format menu. In the context menu (right click) you can find options for 1 and 1-1/2 and 2 lines.
HOWEVER, I believe this is a bug (or poor design). I believe most word processors keep the paragraph formatting and line spacing independent. LO does not do this. For example, if you set your paragraph formatting to Proportional at 120% the document should “remember” this setting. Unfortunately, in LO if you change line spacing to 1-1/2, LO “forgets” the Proportional line spacing. I believe the line spacing of 1 and 1-1/2 and 2 lines is intended for temporary purposes, so that when you no longer need space for handwritten remarks, you can restore the document to its original appearance (in this case 120% proportional).
I don’t know how to put this bug/design problem in the system; hopefully someone else can do it or advise me how to do it if it’s not too difficult.