First of all, i would like to say that having a lvl 80 character with rare/exotic or ascended gear that is tailored for a specific build gives the player much more overall damage and survivability.
As for your questions
1) Necromancers have the highest base health pool posible in the game (along with warriors) on top of that they also have about 13-14k health in death shroud that is constantly “healing”. Elementalists have the lowest base health pool in the game along with guardians and thieves.
Thieves have dodges,stealth and mobility, guardians have regen,blocks and heavy armor. Elementalists d/d have a ton of mobility, and are able to sustain them self with lots of self healing, cc, and the ability to grant them selfs auras and protection boons.
Something to note is that you shouldnt play d/d ele like a necro…it should be plaid as a d/d ele. Dont jump in a fight and try to spam whatever skills are on cool down until one of you is dead… go in, burst him, disengage if needed, apply pressure with conditions, reengage, interupt him with a knock up during his healing.
Pay attention to your opponent.
Also…dont be afraid to fail. Each fight you will learn the strengths and weaknesses of other classes as well as your own weakness. You will also be able to tell if 13k health for you is enough or if you need 3k more. Feel free to try out different gear and builds.
Or just..copy paste something from the web and hope you like it enough to learn to play with such a build.
2) Staff Ele has a ok “single” target dmg, its just that the auto attacks are slow and other skills are more oriented for large aoe groups of mobs.
Staff=heavy dmg, lots of aoe long range
Scepter= Sustained dmg, has lots of utility, good for condition dmg, medium range
daggers= fast paced short range various kind of dmg with a bit of utility
Focus= Defensive skills and utility skills mostly
3)Might helps on all classes and weapons to kill stuff faster. If you have high enough power by using sigils, weapons and gear then you dont need to stack might.
However d/d has the ability to trigger pyromancers puissance trait fairly often and quick and the trait can be used to get might stacks quicker than using any other elementalist skill.
Note this trait also grants might if you use conjure weapon skills and utility skills. Any skill that is activated while youre in fire attunement.
4) D/D is more for roaming builds, water would be much better used if youre following a zerg (i assume you are doing that considering you wont lvl up much if you roam around)
WvW Staff builds can be built in 2 ways:
1) Full glass cannon. Destroy the enemy zerg from the far back of your army with extreme meteor showers, ice storms and other aoes. Can also be used to quickly destroy enemy siege,or put enough pressure on the enemy that is using the siege to run away.
2) Suport damage dealer: Deal good dmg to the enemy zergs while also being able to support your own army with frequent water fields,condi clenses and cc’s while being tanky enough to move with your zerg at all times (call your water fields if youre using team speak, this way your zerg will be prepared and will blast finish your water fields more often and a single water field can fill up the entire zergs health to 100%)
WvW d/d builds can be built in a few ways as well, but most commonly it would be a d/d build that relies on boons,auras and fast spells that a d/d can dish out at the enemy.
There are also some tricks you can pull off like going stealth with a thief, and channeling churning earth while he attracts nearby enemies towards you. Then you flash on top of a player dealing a large critical hit as well as tons of dmg via the churning earth.
You can also share your auras with your party members, or nearby allies if you are roaming in a smaller group. Usually with a auramancer build sharing auras also grants various boons to you and allies.
