1) yes fish are entities and won't spawn just because you throw in a line. You have to go where they are.
2) yes.
3) its complicated. stamina, your skill, the fish level, type of fish, distance all play into it. But generally speaking fish have 2 modes, run and rest. When running they use more stamina to reel in. When resting its easier. Past that its just the combo of the other stuff as to how much stamina they drain vs what you have from your food vs distance to pull in. You can cheat a little by walking deeper onto land so if they get loose, you can just pick them up, but if you do too much of this the line breaks.

4) no, not really. Its tied to whether its in run mode or not.
5) yes
6) yes, and learn to tell their types apart by sight as well. Also bigger ones are harder to reel in.

Fishing is a mess right now. I am hoping for some redos on recent changes, but that should get you started.