Does dualcast double gaining skill level

Making it "easier to cast" is relatively straightforward - it reduces the Magicka cost of the spell, meaning you can cast them more often.

This is similar to how things worked in Oblivion. (I'd link you to the UESP page documenting this, but the Site's still getting hit pretty hard from all the Skyrim traffic)

Each spell has a "base cost", which is modified by the following formula:

spell cost = base cost * skill multiplier * perk multiplier * equipment multiplier * dual cast multiplier
skill multiplier = 1 - (skill/400) ^ 0.65

This means that at 100 skill, spells cost 60% of the base cast (further modified by perks and equipment).

This isn't combat related, but as Affe has pointed out below, increasing your magic skills is required to buy high-end spells from vendors.

This is pretty much the same build that I've been using. I put a bit over half my total points into Magicka, most of the rest into Health, and very few into Stamina. I don't use many power attacks, and when I do, it's to finish the enemies off or stall for time while my Magicka is regenerating. It's working for me so far.

You will get enough perks to fill up 4 perk trees with room to spare, so don't worry about starting into another. I'm focusing on Conjuration, Destruction, Restoration, and One-handed, with a minor amount in Light Armor. Putting a point or two in a tree just to get a slight bonus isn't going to harm your chances of completing other trees.

Even if you completely focus on Conjuration, chances are you won't use every perk in that tree. You might focus on Atronachs over reanimating corpses, or you might not use bound weapons.

Just pick whatever perks you want and don't worry about it too much.