Here's my take...
Sequence #1:
You're free to use the "Vampiric Touch" as the off-hand attack and use the greatsword for the full-attack [channeling], but, IMO, you still incur in the penalties for TWF when using the greatsword. The fact that it's being used two-handed doesn't matter. It's roughly the same as using the greatsword and an additional attack with armor spikes, for example.
TWO-WEAPON FIGHTING
If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. Fighting in this way is very hard, however, and you suffer a –6 penalty with your regular attack or attacks with your primary hand and a –10 penalty to the attack with your off hand. [...]
(emphasis mine)
The way i see it, the penalties apply because of the fighting style you're using (i.e. you're coordinating yourself to deliver two sets of attacks with different weapons) and not because you're wielding a weapon in each hand; as you know, there are some other ways to gain off-hand attacks and they all incur in the same penalties. Wielding a weapon in each hand is just one of them.
If it makes you feel any better, you can still decide which one of the weapons is considered the main weapon (in your case, the greatsword), and you can also choose to strike with your "Vampiric Touch" off-hand attack first. This means your highest penalty for TWF will be applied to the touch attack, so, all-and-all, you still have a nice chance to hit.
Situation #2:
If you full-attack with Arcane Channel, you don't have to recast the spell to use it in the extra attack granted by
Haste. The extra attack is added to the full set of attacks you're allowed during the full-attack, so you arcane channel the original spell in that extra attack as well.
Also on this, i'm not sure you can stop a full-attack action to cast a spell, even a quickened one... I can even assume it's possible, 'cause a swift action is basically a free action with another name and limitations, so i can go with that; but you're using a greatsword for the full-attack, and you switch to the hand to deliver the touch attack. If you did this in a game i was DMing, i would call it TWF and apply the appropriate penalties.