1.) In Factions/Nightfall, yes, you will have to purchase most skills. That's part of the reason quests tend to reward both more experience and more money in Factions/Nightfall than they do in Prophecies. More skills (I wouldn't like to say 'better' seems even some of the skills you can get in Kamadan are very good skills) will become availiable from the various skill trainers as you progress through the campaign.
2.) Yes, you will still be able to pick up any post-Lion's Arch Prophecies quests to learn skills for your secondary class as a Dervish if your secondary class is one of the 6 core classes (Warrior, Ranger, Monk, Necromancer, Mesmer, Elementalist). However, the skill quests that take place before a Tyrian character arrives at Lion's Arch (those in Ascalon and the Northern Shiverpeaks) will be unavailiable to you.
3.) Run speeds don't stack above 33% I believe, though one skill can go over that limit by itself. You can also still stack run speeds buffs in order to counter Cripple or snare Hexes, up to a max speed of 33%, though it's probably not worth the bar space when you could just bring a more universal condition/hex removal instead.
4.) Not really. Dervish is a very effective runner on it's own skills anyway. There's a little elite skill called Vow Of Silence which can be maintained permanently, round the clock, and makes any spells unable to be cast upon you. This of course means that the enemies won't be able to target you with snare Hexes to begin with.
The downside, is that it prevents you from casting spells yourself too...which means you won't be able to use any Monk-based condition removal, nor some of your own run spells. Therefore I'd recommend either a Paragon or Ranger secondary to remove Cripple from yourself should it fall upon you via Signet (Remedy Signet or Antidote Signet, respectively) and there Shout and Stance based run skills which you'll be able to use despite Vow of Silence, being as they aren't spells. You'll also get some extra healing options from Paragon, and some extra damage mitigation options from Ranger. You could also go Warrior secondary for some anti-knockdown or some other run stances, or Assassin for some
very good run stances, and also shadowsteps if you feel like using Shadow Form for spell immunity instead of Vow of Silence.
Of course, if you have Eye of the North and don't mind playing a little ways into it, another simple solution is to get the Norn PvE only skill "I'am Unstoppable!", removing any need for a secondary at all (though going Paragon secondary for Enduring Harmony might still help) which is a shout (so can be used through Vow Of Silence), offers immunity to Cripple (and it'll remove it if it's already on you), immunity to knockdown, and extra armour as a bonus, all in one skill.
Dwarven Stability, another PvE only skill, can also be very helpful for running, and can be used with any profession. It's especially useful with some booze to go with it.
Hope that helps.
