What should I do if I’m locked out of wp-admin?

Connect via FTP. Rename the /plugins/ folder to /plugins_old/. This disables all plugins. You should now be able to log in.

How do I undo a bad plugin or theme update?

If you have a backup, restore it. If not, install the WP Rollback plugin (if you can access admin) to revert to the previous version. https://wordpress.org/plugins/wp-rollback

Is it safe to delete .htaccess?

Yes. WordPress will regenerate it. Rename it to .htaccess_old and then go to Settings > Permalinks > Save Changes to create a fresh one.

How can I troubleshoot on a live site without downtime?

Use the Health Check & Troubleshooting plugin. It isolates your session so only you see the vanilla version of the site while visitors see the cached, live version. https://wordpress.org/plugins/health-check