Is it possible to reach level 50 with quests?

I haven't touched PvP so far and I didn't really grind either. I don't think I'm too low for some specific area. However, I didn't skip dungeons. Don't do it. They're not as hard(core) as in some other MMORPGs.

The smallest dungeons - often referred to as "Delves" - can be soloed. You typically have to fight through some enemies, and then kill some boss character. This will get you some loot, typically another short quest, and one skyshard as a bonus.

The medium sized dungeons - public dungeons - are open for everyone as well, but you'll most likely want at least some help in them. Grouping is still optional and you can get nice rewards, skyshards, and gimmicky items (for Ebonheart Pact, have a look at Crowswood right below Davon's Watch on the coastline).

The biggest dungeons - group dungeons - are instances made for one group. Those usually offer one storyline and one or more quests introducing the dungeon. These require a full group of four players, but once again they're not that hard(core). I've completed them several times now, everytime with a random group that wasn't planned or anything.

Also: Never, ever pay for some addon. Most of them are either stolen and/or don't provide any information not available for free on the net. The one you linked seems to be the latter. If you've found some free addon and want to support the developer, donate a bit, but never pay for something like that. That's also against the TOS and unless that developer got the okay from Bethesda, this looks very fishy to me.

Questing is - in my opinion - not enough.. I'm currently only lvl 23 since I don't have a lot of spare time atm, but most quests I get/do are beneath my level. I'm writing it in an answer because it's to long to fit it in the comments. What I do in every area is:

1. All quests I find / currently have
2. small dungeons (skull and bones on minimap)
3. Go to every unfinished map marker (when it's black, you haven't done everything yet)
4. public dungeons..

You also get xp from exploring, similar to other games and unlocking chests.. I find the combination and variety of these different things to keep the game very interesting, I always find something to do. So I think PvE only can get you to 50 if you do everything of the above correctly but it'll probably be a close one. I do recall reading somewhere that pre-release they have been focusing more on the first X levels to keep it interesting and have enough quests and that they will move their focus on content and quest lines on the later levels after the release. But I can't find the source anywhere so...

My level isn't high enough to sufficiently support my theory.

I got a lvl 33 Khajit assassin/dualweapon/ranger. And still got quests in Greenshade (got 2x29lvl and 1x30lvl quests there) I discovered the 3/4 of that map. I love discovering the world, find more quests and searching for ingredients. Doing research, deconstruct items I dont needed (not selling them). Sometimes go with friends or killing bosses, because this is an MMO-RPG and we like to have fun. Doing quest by quest is too boring, there is a whole world waiting for me to discover. I discover everything, and do everything on the maps (quests, bosses, dungeons, skyshards, lorebooks). So it is easy to stay in your level, for me its a little hard because when I catch my level in quests, a new map open to me, and I go to discover it, so I get more and more experience and I'm always over of the quests level.

I'm a level 38 Khajiit from Daggerfall Covenant. I had never done main story quests.

I completed all zone quests from starting zones to zone 1 and zone 3 (Stros M'Kai, Betnikh, Glenumbra, Stormhaven & Rivenspire). All the zone public dungeon, 3 party search dungeons, solo dungeons, dolmens are all done for the said zones and levels. I skip one or two delves, the skull thingy. All achievements for zones are done, and I can confirm Paralytic's statement. There will always be 3-4 quests extra as compared to achievement required amount of quests. (referring to zone completion achievement) I make sure all my map markers are white with exception to the skulls.

Correct me if I'm wrong, I noticed each zone is separated to 4 level tiers. Each level tier has 3 alternative quests/area/marker. If it's an area/town, expect 1 extra mini quest. (64 for completing Glenumbra, I am sure I did 72 or more) Zone clear achievement accounts for clearing areas, mini quests/errands are bonus. For example at level 33, you have 3 mini quest lines or area to clear, it will be always on the way to your main zone quest marker. If you stay on the roads of the map you will definitely discover the markers and way shrines.

I then participated in Cyrodiil for 2 daily 20 players quest, and vamp quest. I arrived in Alikr Desert at level 36, done the 3 party search dungeons and Alikr zone public dungeon, I became level 38 before I start zone 4 quests. This actually allows me to skip zone 4 and head straight to zone 5 Bangkorai if I wish to do so.

I noticed that if you complete all the zone quests, you'll always be 3-4 levels higher than the quest mobs. and if you do all the dungeons, you'll be 5-6 levels higher. At this rate, I'm expecting to reach 50 by quests alone. (i.e. exploring all zone content)

TL;DR read the bolds.

I am VR1 now. And I still have 28 Quests in Rift and Coldharbour to do. I did all but 2 group dungeons and as many open world and solo dungeons as I could find. I did two short stints (obout 60 min) in Cyrodiil.

It is importend to do quests at your level because you get more xp for killing mobs at your levvel than for killing underlevel mobs.

Therefore the Answer is Yes! Anybode telling otherwise doesntt know a thing.