-You can't channel petty magic (partly because the casting number is always 0, which makes channeling moot)
-Yes, you memorize all petty mgic, there are none in the grimore.
-Learning new spell costs are a little poorly explained, so i'll try to simplify it for you.
When you take the petty magic feat you instantly get your willpower bonus (which is your willpower divided by ten, or just the tens column if you're lazy with math) of petty spells. petty spells are always memorized for purposes of the game's system, and do not require a teacher normally. you may, between adventures, purchase new petty spells using the costs listed in the talent.
whenever you learn a new petty magic spell, you calculate the cost based on the number of spells you already have. so as long as your spells are the same total as your willpower bonus, you pay the cost of learning a spell x1. Since you get your willpower bonus in spells for taking the feat, the first additional spell will always be at the X1 cost (barring you losing willpower somehow). you would only pay the cost at a x3 multiplier once you have more than three times your willpower bonus.
-Necromancy is a dark lore, and is intended for NPCs. if the GM wants he can let you learn it anyways, and the rules explictly state you can learn an arcane lore AND a dark lore.
-The dark lores are only stop-gaps for the GM tou wrangle with while he's making NPCs and things like lore bonuses have been overlooked to save space (which they used mostly to remind you the rules are optional 80 times)
if you want more dark magic, i have made an article on Fimir which contains a set of spells for their use at the end. finding a scroll or two in a forgotten crypt in a bog wouldn't be out of reason. you can find it in the homebrew part of the forum, and i hope you'll leave some feedback if you use them.