Stana Katic (if she wasn't already otherwise occupied with Castle) would make a great Lois. She's a few years older than Cavill, but she tends to be a bit of a chameleon and would easily make it work.
Missy Peregrym, Rachel Bilson, and Mila Kunis might be decent choices, too.
(Years ago I liked Rena Sofer for the role, but she's 42 now against Cavill's 27. That's not going to happen.)
I like your choices. Missy Peregrym never occurred to me but she could be quite good. I really like her in Reaper.
I was also going to suggest Stana Katic or Mila Kunis.
As to the casting of Cavil as Superman/Clark Kent, I haven't seen any of his work but I'm going to assume that the fact he was in The Tudors bodes well. Also he totally looks the part.