Is the checksum still wrong? If so and you've done the above - and thoroughly checked that ALL CK2 files and directories are manually deleted before reinstalling - I can only guess there's something wrong with your machine or your Steam client or account.
Try it again, but this time after deleting everything disable your anti-virus software before the reinstall.
If that doesn't help, I'd remove the Steam client entirely, manually delete ALL leftovers, then install it fresh - preferably NOT under the system path c:\program files\ .