mastering a skill vs just passing many?
is it better to master each skill as you progress or to just complete it to unlock the next level? or is it completely subjective and is dependent on a case by case basis?
There are two ways of mastering a skill now: to do all the suggested "real world" translations or to test out. The first way is much more useful. However, when you are still a beginner, it is extremely hard to translate unadapted texts, so I'd suggest to leave the lessons as "learned" and come back to them later to do the translations and "master" them.
As for mastering vs going to next lessons, it's up to you. I usually try to gain some confidence with the new words before I move on. This does not imply mastering because practice does not change the unit progress.
As far as I get it from the discussions, the developers are deploying a new method of progress tracking, and the lessons soon won't be mastered once and for good. They will have indicators of word strength.