Regarding the inability to log in, I did find that some behaviors had inadvertently changed, introducing a "Catch 22" situation. I just released version 0.9.0 to fix the problem. Now, when users choose an insecure password during the reset process, they can pick a better password right on the spot.
Please re-run the manual data removal queries I showed you, above. Of course, you must change the "wp_" table prefixes to match your table prefix. Please make notes of the results of the queries so we can keep track of their effectiveness. If you have further problems, I'll want to see them.
Then upgrade the plugin and reactivate.
On the sending emails for 0 failed logins problem, that's totally bizarre. If the problem persists, please send me those removal query results.
Please keep me posted.