I may not know much, but I know what many others around the forums have said.
From what I've read, it's best to max all the skills for dual axe, though again I may be wrong. The popular sub would likely be cleric, since dual axes take more punishment than 2handed and 1handed warriors, the former being able to kill faster and the latter having a greater tanking ability.
To answer your questions:
1. Dual axes in my opinion aren't worthless. They can be an integral part of euro parties and from videos on youtube, they do a fair share of heavy damage. Whatever character you want to do is up to you, as long as you enjoy it ^^
2. I'm not sure on this one, but I'll go with yes. It might be by adding 1handed skills like shield trash and such, as I've seen some 2handed guides recommend that. Cleric skills will come in handy here.
3. They have their fair share of wins and losses in pvp. I've only seen them in youtube though, not in game =/
I believe you don't see many of them because they're the moderate warrior. Between 1handed's tanking and 2handed's crazy damage, the dual axe lies somewhere in between. I've also seen people complain that they feel the dual axe skills are gimped... But don't let that discourage you, they do look amazing =D
lol, I'm sorry I can't help there. There's warrior cleric guides you can look up in the forums that'll give you the recommended skills. I'm not familiar with euro since I have chinese characters only that I'm messing around with, but off of what I remember you want to get recovery division, which is equivalent to a 5 second pot delay. Apart from that, I'm not much help anymore =/
Well... since you're doing Warrior ... hehe ... You should Max all skills from Warrior tree ... you can take any weapon you like ... Axes , 2h Sword or 1h Sword ... I perfer 2h sword really ... They are pure sex ... hehe ... anyway for submastery as cleric ... the skills you need are
Healing Cycle/Orbit , Holy Word/Spell , Ressurect , Recovery Division and Bless Those are a Must ... no matter what weapon you take ... anyway ... as most people say ... Dual Axe sux ... so up to you ... I'd make 2h with 1h and sub cleric ... enjoy