Yes to question 1. Question 2 is more complicated. In general, for endurance riding the more muscles you use, the longer you last. For speed work this doesn't seem to be true, possibly because coordination speed is a limiter. While some say that strength is a limiter, ever look at the legs of an elite track sprinter? They look nothing like the legs of a grand tour climber. So it depends on what you want to do. The principle of specificity applies: you get better at what you train to do. IOW your question is too general.