Questions: self drilling/ tapping screws for chassis work

rhagfo mentions that this thread is over-thought. That may be true for folks who are already experts in working with metal. But a lot of folks on this forum (like me) are ordinary do it yourselfers, who do not have any expertise in that area.

So that is the reason for all of the questions. I'm ignorant, and I am trying not to mess up my project by buying the wrong screws. As I told my students over a 38 year teaching career, "The only dumb question is the one you don't ask!" ;)