Apologies if these have been answered. I can't help myself 
- Faith was made a slayer when Buffy died the first time. When she dies in Season 5, no mention of a duplicate Slayer, correct?
When Buffy dies the first time round the line passes to Kendra and then on through her.
- The Principal in Season 7 talks to Buffy about how he's looked through her file, why is her response not, 'you creepy fuck, what a breach of privacy, I left school 5 years ago'?
Yeah, that's super creepy.
- Neither Buffy or Willow have regular jobs (Willow is never seen in employment). Who pays for Willow's flights to England for rehab, their weapons, various house repairs (don't tell me Buffy could get insurance for accidental damage / civil unrest with her claims history), Dawn's school books etc.?
I think part of the funding comes from the watchers council, particularly in the early series. Willow's parents are funding her through college and Giles has money too. Also, don't forget that Xander handily learns over night how to be an excellent carpenter, electrician, plumber and decorator 
- When Spike goes to get himself restored to his former self: why exactly does that Demon con him and actually give him his soul? If it was a good demon I could understand it's ultimately setting him on the road to redemption, but it's not.
It's like the old 'be careful what you wish for' thing. The demon does it because it's not the interpretation that Spike intended. He doesn't care about the consequences - good or bad.
- Why did Joss Whedon make Angel Irish, when it must have been obvious within 5 syllables that David B's Irish accent was the worst thing in the world?
the same reason he made Spike English 
- Kennedy: I was all for Willow finding someone else after Tara, why on earth did they make that person such an utter knob? Is she meant to have some redeeming features / be perfect for Willow and the actor ballsed it up?
ugh, couldn't stand that relationship either
- After Joyce died, why didn't Buffy's dad make a single appearance, surely they could have found some reason / decent plot to make this happen, it's so clanging for me that a parent, no matter how absent, wouldn't even stop by once after his childrens' mother died suddenly.
he is increasingly absent over the years. I think this does happen. It happened in my family.
My question. The much prophesied vampire with a soul. Is it Spike (who fought for and won his soul) or is it Angel (who had it shoved back in against his will and gives it up at the drop of a hat)?