To better answer your question about the enemy level range shown on missions:
Your Mastery Rank doesn't correlate with enemy level, and neither does the rank of your equipment.
Mastery rank is used to meter out what equipment is available to you. You might get parts for a weapon above your mastery, but you won't be able to build it in your foundry. There are a few weapons that you can get 'early' because they come from events and drop straight into your inventory. Generally speaking, higher mastery weapons will be have a higher build ceiling.
Enemy level is a representation of difficulty. Which levels are 'easy' or 'hard' really depends on your skill and build, so knowing what range your build specifically starts to struggle at is important.
Equipment rank is a measure of how heavily you can mod it. A rank 30 weapon has a base mod capacity of 30. Adding an Orokin Catalyst will double that to 60. Using forma to polarize all eight slots, which would require fully ranking the weapon nine times over, will halve the capacity rounding up, so a 15-drain mod halves to 7.5, round to 8, so your total max capacity is around 120 with a lot of effort. You'll almost never go that far, as it isn't real necessary and locks you into those specific polarities until you put in the effort to change them. You also get your mastery rank as a 'minimum' capacity. A mastery 10 player will have 10 capacity on an unranked weapon (20 with a catalyst) until it hits rank 11 and gets its 11th capacity point.