Is it possible to reach level 50 with quests?
Long story short, yes you can make 50 on just PvE.
I am currently half way through Veteran Rank 1 and I had no problem getting to level 50 with just quests. It was close I will say, being on the final few quests before finishing coldharbour. I will also mention that a lot of people are having issues with finishing coldharbour and only being level 45-49.
I pvpd maybe once during my journey to 50. I did the beginning questline that gives away 2 skillpoints, and then continued to finish the 20 kills quest. so maybe a small 30-40k boost of experience, so nothing huge. I did every quest I could find, only not finishing Shadowfen (by 7 quests) and The Rift (by one quest) because I couldn't find them. Also missed one quest in Coldharbour because it is bugged (I keep going back to it every morning as this seems to fix most bugged quests, I also wake up at 7 am everyday so it is pretty quiet and I think the servers don't freak out as much.) I know I missed these quests because the achievements tell me so, although I have noticed in Deshaan and Stonefall had more quests then the achievement said there were.
I would say a lot of peoples issue with hitting level 50 on times is that they only do 70% of the content. As Exploring, public dungeons, private dungeons, and side quests is atleast 50% of what you need to level on time. While the main storyline quests and the quests you happen to walk by doing the main storyline is the other 50%. So if you are worried about not making 50 on time make sure you are doing atleast 90 - 95% of everything so that you don't find yourself spamming coldharbour for a grind party.
I will finish by saying I also found myself multiple times 1-2 levels below the mobs and quests, it was confusing me a little at first but do not worry. Take your time, enjoy Tamriel and remember to take some time to just explore, discover and finish some things outside the beaten path.
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:
- All quests I find / currently have
- small dungeons (skull and bones on minimap)
- Go to every unfinished map marker (when it's black, you haven't done everything yet)
- 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.