Good explanation.
Why should one thread a machine with the presser foot up? What happens if the presser foot is down instead of up?
I do find the old saying - "righty, tighty; lefty, loosy goosy" - helpful to remember which way to turn the screws.
When I've turned the tension adjustment screw on the bobbin holder, I've found that even quarter turns can make a noticeable difference in the stitch appearance.
I have a Singer 237 (I think early 60s lower end machine) and the tension changes all on its own - so I have to tighten it up every so often.