Have you guys been clearing each zone 100% then moving on to the next, or are you just questing through until the main quest line takes you to the next zone?

Questing through. Though I am only level 18 haha. Just take all the quests you can get, and generally you can get them done more efficiently since you can do a bunch of things on one outing, then turn them all in back in the same area.

ideally if you are farming, you should be +5/-5 within the monsters there.

When I quest, I always do +2/3 higher level than I am.

I clear a whole area then move on to the next.
The only quests I skip are the super annoying ones.

I do the same, clearing the area 100% and moving on to the next area. I skip quests that are bugged and never come back :S even when they are fixed (looking at you "The Ghost of Westtry")

I have always played every ES game for exploring, so I 100% an area no matter what. When the bug fixes came out for the broken quests me and about 10 other lvl 40+ were in lvl 8 and 20 areas finishing the grey quests lol

100% all quests and skyshards

Is it possible to not be able to do 100% quests? Can you out level quests where they won't show up for you then you're doomed to never finish?

no, quests always show up, regardless of your level.