I created my Ask Ubuntu login with my Google login, but if I try to manually login with my Google email, and regular Google password (rather than using the "Google" login button), I get "wrong password" (or something similar). If I can manually login to Ask Ubuntu with my Google email, and regular Google password, then I can use the LastPass entry to login. Why does this not work? I am using the correct password as evidenced by successfully logging out of, and back into Google manually, AND logging into Google using LastPass.
You'll have to login to Google first as you have discovered and use your LastPass creds there, then use Google OAuth / OpenID auth to authenticate with SE.
Your LastPass credentials being put into the email and password boxes on the logon page for StackExchange / Ask Ubuntu would be for StackExchange created OpenIDs, if you had registered without using an external logon identifier (Google). That's why you get the password errors.
TL;DR, you can't use the email/password boxes to logon with your GMail credentials, because that's only for SE OpenIDs if you only register on SE and don't use the external authentication from Google.