1) You can still learn Sun spells, but only at 1/2 your current level in Suncraft. Thus, if you are 8th level Suncraft, you can only learn 4th level spells. This restricts you to 6th level spells (12th level Suncraft) or lower.
By changing to Warlock, yes, you get the ability to learn spells at the current craft level once again.
2) Assassin: Moon and Fiend magic. Go that route.
3) Once again, on the Moon spells: it'll be at 1/2 the Mooncraft skill level, like the Sun spells.
Cheers!
-Sazerac