I have a couple PC built NPCs that are adversaries to the PCs' goals, but in the next session there could windup a significant conflict between either a 3rd Tier Moon Druid or a 3rd Tier Mundane Ranger (Hunter). Are there any bits of advice on how not to get bogged down in minutia with the amount of skills/spells while still maintaining either of these individuals' builds for story purposes?
I'm not worried about them dying. They will eventually. I'm more concerned about how to manage a combat encounter when the opposition has wider spell choice (the druid) or maneuvers and pets (the ranger)?
I have found "good enough" simplified conversions of PC classes work well for NPCs. Don't be wedded to strict use of the PC rules, feel free to use shortcuts & altered versions of class features to make running them easier at the table. The 3e mindset of "must design NPCs using PC classes" makes for NPCs that are a PITA for you to run as DM. There is no player who is going to criticize you as DM for changing minutiae. A better mantra for 5e might be: "design NPCs using PC classes as inspiration." That thinking seems to apply to the sample NPCs in the Monster Manual. As long as the NPC *feels* like the class in question I think you're good to gol
For your shapeshifter moon druid, pick 2 forms in advance from the Monster Manual (CR 4 or less if a 12th-14th level druid) and flag those pages for easy reference. Pay attention to how Hit Points & Damage work for shapeshifting - also remember if you want to play up the shapeshifting, you can have the Druid burn spell slots on Combat Wild Shape to heal as a bonus action. As with any spellcaster, familiarize yourself with spells beforehand (and if you did CR calculations for the Druid, you probably did this anyways when calculating DPR over three rounds as the DMG suggests).
For the hunter ranger, there aren't any maneuvers to keep track of (maybe you're confusing it with the Battlemaster fighter?) and they don't get an animal companion, so I'm not sure what you're thinking there. But in general just treat animal companions as another monster in the encounter allied with the ranger - no need for special rules. More generally with the ranger, you can omit a lot of class features from an NPC build including Favored Enemy, Natural Explorer, Hide in Plain Sight, and maybe Primeval Awareness. It's not that the ranger NPC doesn't have these things, it's just you don't need to include them in the stat block. Omitting them makes handling the NPC at the table easier.
One last word: A "third tier" (level 11-16) NPC who is not throwing about area effect spells is probably going to be around CR 4. I'm basing this on a bunch of 12th level NPCs I created following the DMG maths and most of them turned out to be CR 4. What I generally found to be true is that they had low defenses and higher offenses. If your party is just facing one NPC designed with PC rules and they are good about focused fire, your NPC might not last more than a round or two. YMMV.