Well i assume by your first point your asking if once you buy it you can level all of your wizards up to 50 without buying it again. This is NOT true. You buy it on the character you want and that is the ONLY character that gets leveled up.
As to your other points i would say no its not worth it. I got it on my ice and i regret because its really hard adjusting to the new character without slowly leveling up, i dont know what i spells are best and so on.
I think you should only by it after your second casually leveled up wizard. It's better to have an understanding of the varieties of school spells and what they can use.
It helps for those slower schools at the start such as ice or life. Death can be tricky to learn at 50. As well, storm and fire can be difficult if you're new to low health, higher damage, faster attacking playstyles.
It's nice to skip the first arc, but you could honestly level up from the start and have more preparation going to level 50. I used it on my death wizard and I wasn't familiar with the playstyle at all. I casually leveled up my fire, but I should of used it on him as I have a storm and the playstyle was similar.
Honestly, try it out on a school you know you'll might like and get going!
Hope to see you all in the Spiral! Nathan GreenEyes 120 PvP Commander Nathan GreenEyes 117 Hot in more ways than one (lol) (Currently questing through Mirage) Nathan GreenEyes 110 Couch Potato Hoarder Genevieve HexPyre 100+ PvP Warlord (Currently questing through Khrysalis)