I am sorry if these questions are basics, but I am not sure I totally understand the basics of how a laser works and the books I read are not totally clear for me, so any explanation would be greatly appreciated. So assuming we have a plane mirrors cavity and a gain medium in between, we create a population inversion in the gain medium, which initially creates photons by spontaneous emission and the photons that are produced parallel to the axis of the cavity gets amplified. The amplification is produced by stimulated emission due to the fact that the photons produced this way are in phase with the initial ones. So in the end from one spontaneous emitted photon you have a lots of photons. Here are my questions:

1) These spontaneous emissions can happen many times and at different locations inside the gain medium. But these spontaneously emitted photons are not in phase with each other (but they have the same frequency). So if 2 spontaneously emitted photon are out of phase by half a period, even if they get individually amplified a lot you would have 2 huge bunches of photon out of phase with each other. So the 2 electric fields would be huge, but they would cancel each other, so the output of the laser would be zero. Of course perfect cancelation is unlikely but I feel like overall, given the huge number of spontaneously emitted photons there would be lots of cancelations. So even if the amplification of one photon is super efficient overall it seems like this is an inefficient process due to this out of phase problem. What am I missing here?

2) Assuming there is just one bunch of photons (basically ignoring the problem in 1) ), the laser output appears when these photons hit one of the mirrors. So this happens every 2L/c seconds, where c is the speed of light in that medium and L the length of the cavity. So the laser created will appear as a pulse. However, I read that there are continuous wave lasers, too. How does that work? Doesn't the light still need to go back and forth between the 2 mirrors, how can you get a continuous output? 

Thank you!