Question: Why did Stuart continue trusting the Canary when she lied to him and the whole family?
Answer: Because she wanted to be safe from the evil Falcon.
Question: This isn't really about this movie, but I could have sworn there was a third Stuart Little movie that was all CG. Does this actually exist or am I making things up?
Answer: Nope, you're not making it up. There is indeed a third movie in the series, called Stuart Little 3: Call of the Wild, which is, unlike its predecessors, completely computer animated.
Question: Since George obviously doesn't like soccer, why is he doing it?
Answer: Probably because he could see that she was hurt at what she did and also because earlier in the movie, he said "When you're friends with Stuart Little, you're friends for life". Also if you wanna look at it another way, him helping her could be payback for her saving him from the sink. It's just a long term friendship they have.