1. Technically, the Wizard and Sorcerer templates require both. However, Templates aren't classes, they're intended as a starting point, not a straightjacket. If you want a character with only Evocation or Thaumaturgy, you can definitely make one. They won't technically be either of those templates, but are still likely to be referred to as a Sorcerer so who cares? Making up and modifying templates to better fit your game isn't just permitted, but actively encouraged (see YS p. 72).
2. Channeling is almost as good as Evocation for practical purposes (both give 2 Item Slots and the full range of basic Evocation effects), all you get from Evocation is two additional Elements, and a single specialty. You also get the ability to get Refinements...but if you have Channeling and the Refresh to do that, then you can just upgrade to Evocation. Why wouldn't it be almost as expensive?
2. Uh...what are you asking? If someone wants to spend 6 Refresh on (for example) Thaumaturgy and Unseelie Magic, and have a good explanation, more power to them.