When I was setting up the 970. I enabled ECG. That required two factor authentication. Now, I'm stuck with that and no way to go back. Anyone have a similar situation?

It's really not that big a deal. Its not reversible as far as I know. 

The Garmin connect app seems not to require any new 2FA once logged on. That's how it behaves for me.

 The browser seems to have a expiry time for the 2FA, unknown how long.10 days or so it seems. Anyone knows?

For some month ago I needed to use 2FA every time I logged on the connect in browser. That is gone now.