Now that I’ve finished a project, how can I change/decrease the first line indents in the entire document, all at once? EXCEPT I want the first LINE of each section NOT to be indented. Is this possible, or do I have to do it manually, throughout the entire document?
Why not do this in the compiler? In the Formatting compile option pane, click the Options button in the upper-right area, and note the “Remove first paragraph indents” checkbox. Select that and the criteria for how this should occur. Submit that, and then while you are here, click the “Override text and notes formatting” checkbox to the left of the Options button. Now in the list below, select the text file icon (assuming text files are where you wrote your content), and use the mock editor below to set up how a typical paragraph should look. Everything you see here is how it will appear in the final copy.
I suppose you could do this in the editor, but it wouldn’t be easy. Adjusting the indents wouldn’t be too hard. You’d just need to change your application formatting defaults in the main preferences, and then use Documents/Convert/Formatting to Default Text Style on a selection of files to update their formatting. That will do for the decrease in indent. But to remove the first-line indent for each initial paragraph, well you’d have to go through by hand and set that up.
I’d really consider just using the compiler’s tool for that, that’s how the program is designed to work. Write however you want; publish the way you need. 
Congratulations on finishing your project, by the way.
Thank you so much–very helpful!