don't look at it as "one pokemon vs one pokemon" when its actually a team vs a team
while its true that only one gets active at a time, in the case of singles, you can always switch to another member of the team, switching to a better suited team mate is the basis of competitive play
can look at it as a decision tree i guess:
do i switch?
y: switch to what?
n: which move i use?
the other questions, well, if you have read as much as you claim you should know by know, if that didn't help then the hands-on-practice approach may give you better results: off to matchmaking you go!