A) Is it true that working up hill/lifting weights builds up lactic acid
A) Using your muscles use energy in the form of ATP. Basically there's 3 main energy pathways. Primarily, they are fueled through (1) creatine phosphate (phosphocreatine) transforming ADP back into ATP. Once this fuel is nearly used up the body starts to utilize (2) glycogen stores (glycolysis) in the muscles and yes it does break them down to pyruvate/lactate which can be utilized for the (3) citric acid cycle & oxidative phosphorylation in mitochondira.
So essentially the answer is yes. These systems are not separate from each other; it's not like they turn on and another turns off. They are relatively fluid which means even if the majority starts coming from phosphocreatine, there is still energy coming from glycolysis and oxidative phosphorylation. On the other hand, you will generally feel a lot of lactic acid burn usually within about 1-5 minutes depending on the exercise. The higher the intensity (and thus more energy you're using), the quicker you'll feel the burn.
B) is it true that working down hill, lowering weights stresses muscles and makes them grow.
B) Running downhill is primarily an eccentric exercise like pullups lower down from the top position are. If you're relatively well trained, then just doing that probably won't do much for you. Eccentric type contractions are the stronger though so utilizing them in a training program can be effective like if you were working towards a one arm chin then one arm negatives would be good. For muscle growth, the eccentrics you'll get from running down a hill probably won't help you at all unless you're completely new in which anything will help you. You'd be better off doing really heavy squats, DLs, oly lifts and such.
More sets less weight means muscle tone?
Myth. Muscle "tone" is built by either losing fat or gaining muscle which are mostly dependent on diet. A good training program can help stimulate fat reduction and/or muscle growth (doing both at the same time is generally not optimal though). For example, metabolic conditioning programs like crossfit WODs and heavy lifting with large compounds like squats and DLs help stimulate testosterone and growth hormone release which promote an anabolic environment for muscle growth and catabolic environment for fat cells.
Generally speaking, the best environment for muscle growth comes with lifting heavy for many sets and low reps.
More weight less sets mean muscle growth?
Myth. Muscle growth is primarily determined by stressing a muscle enough that it will adapt. Again, like the last question generally speaking the best environment for muscle growth comes with lifting heavy for many sets and low reps.
I would HIGHLY suggest you read the link in my sig on how to build a workout routine. I cover most of these concepts pretty extensively explaining about the repetition spectrum and intensity and various programs.
