Wizard is easier to play with than warlock. But Warlock can stun and stuff, making them very powerful in the hands of a knowledgable person. Basically, if you have no idea what you are getting into, go wizard. If you know exactly what is going on, go warlock. Or get warlock and sub wizard, for situations that warlock doesn't work well.
1. Can Warlocks solo?
Yes they can. Warlock deals decent damage, but they do kill normal mobs slower than wizard. Party mobs are untouchable if you are soloing.
Extra) In party hunt, warlock spells' range is pretty short relative to wizards', and their multi-hit spells actually draws a lot more aggro than wizard 's one nuke.
2. If they can, is it efficient? Seems like an awful lot of work for one kill - laying down two de-buffs, and then casting your nuke?
Depends on your defination of effective. Wizards kill normal mobs fast thanks to their high damage nukes. When you are trying to take out a party giant, Warlock as just as good in damage, and also benefits from the ability to stun the party giant. Keep in mind tho, that if you are hunting mobs 5-6 levels above your level, the curse effects might not work that often.
Extra) Many skills in the warlock line are useless for full int. For example, the life stealing skill very rarely save you life after you are level 50+. Once again, be knowledgable about which skills you need and which ones you don't are important. Some skills are useful for full str warlock only (e.g. the nuke reflect skill), and some are not useful at all.
3. Do Warlocks level fast? Gaining experience should not be a problem in party hunt. The biggest problem is to not giving up the exp you just got by dying. But all high int characters face this problem, not just warlock. Unless you want be make a warrior/warlock, and use hexes on the side while using melee damage as your main thing.
Extra) Subing bard for Noise and Moving March will increase your survivability
4. What advantages do Warlocks have compared to Wizards?
AoE hexes, excellent in PvP with stuns and stuff, unique as in a guild of 50 guys, you will probably be the only warlock

.
Extra) Warlock spells gets casted extremely fast, unlike wizards who have to stand there for 2 seconds for the nuke to come out.