a: I don’t recomend to follow the original order of the game. The leveling arch in the first game is rather steep compared with the other campaigns, and even though they are placed behind eachother in time, they are stand alone campaigns and story’s.
B: As Kisanth said having access to the right skills is important.
This is what I would do (without spoiling) to get the maximum understanding of the lore without making it too complicated for yourself.
1: Make a character in prophecy’s. Press m and you see a small section of the world map. After a very interesting cinematic, when you press m you see the full world map (wich is very simular to that of gw2).
2: Start a new character in factions and start playing the game. When you come to an outpost called the marketplace run your character to kaineng center. just outside you find your first hero. In the town you can get henchman and your first hero to get a quest to get to EoTN and get 3 more heroes. Leave this story alone for now and return to factions.
3: Play through factions the regular way. After a mission you can change your secondary profession giving you the abillity to get proper skills for your heroes and capture elite skills for you and them.
4: After finishing factions you can go to Nightfall (with the same character). Here you will unlock more heroes and you should have more then enough resources to give them proper builds (set of skills and attributes).
5: Now go to prophecy’s. On arrival I advise to go back to the starting town (the one you left your first character) It takes bout 60 minutes to run/fight your way there. You can also hire a professional runner who can take you there in less then 30 minutes. Use the wiki to play through the first missions in the original sequence. After Lion’s arch the story-quest will take it overand you can follow that again.
6: After prophecy’s it is time to finish Eye of the north so fo that.
7: Now there are some smaller parts wich you can play in the following sequence: War in Kryta, Hearts of the North and Winds of change.
8: Last of all you could play through the elite area’s. Sorrow’s Furnace might interest you, and is actually easy.