The mana issues arise from long duration fights where there can be little downtime to compensate for the MP spent (i.e. raids and trials). Since dungeon bosses aren't generally long encounters and you get downtime inbetween trash packs then mana issues are less likely to be a problem at your level.

The issue with Ruin II is since its an instant GCD, you become locked out of doing other skills immediately, unless you have an oGCD (e.g Fester, ED, Spur) available to fill that gap with. Ruin on the other hand has a cast time roughly as long as the GCD so by the time the cast ends so does your GCD, giving you the freedom to cast again.

The point about melee range is that as you use Ruin II your character will use an autoattack, so it works out as 80 potency + AA damage. If you use Ruin II at range and you weren't forced to move or did an oGCD after, then you wasted extra mana just to do the same damage as a normal Ruin.