OP here with an update: So, the other day I decided that I want to take a bus or train from Compostela (when I finish) to Porto and explore there for a couple days. This is exactly why I only bought a one-way ticket from the US to Paris. I didn’t want to be held to a schedule and I really didn’t know an exact date for my return. I haven’t even thought about HOW I was gonna return home—I wanted to cross that bridge when I got to. However, all the thinking about whether I wanted to “take a risk” regarding Immigration and passport control in France letting me in without a return flight, got me a little anxious. Understanding how all Customs or Immigration want to do is simply know that you won’t overstay your 90 days in their country. I went online and found the absolute cheapest, no-frills flight from Porto to New York I could find on an arbitrary date in the month of June and booked it. I even paid with points so it didn’t cost me anything. Now I can show Immigration or the airlines my “supposed” return flight. I plan to not use the ticket and, basically throw it away. It was a non-refundable, no-changes-allowed ticket anyway, so I don’t mind wasting the points.
There! Check that little concern off my list. Buen Camino!