Hi Scott,
I think this may be a deficiency that no one had reported yet, i.e. when updating the password on an existing camera, we don't update whether to sign in with Google or with Nest.
To fix this for now, I think this should work.
1. In the Camect UI, disable and then delete all of the old Nest camera windows attached to your old "sign in with Nest" account
2. Restart your Camect device (home settings > show advanced settings > restart)
3. Sign in from the Nest sign-in window, or, if it doesn't appear, home settings > device > add camera, set type to "Nest" and click "add" to sign in.
Note that at the moment, Sign in with Google requires using either no 2FA, SMS, or "Tap your phone". In the next release we'll also support Google Authenticator, but it's not available yet.