I wish to deactivate my plugin, but WITHOUT using update_option() or any other core WP function. What's the best way to go about this? Direct DB connection and modification?
The simplest way is probably renaming the plugin file or folder.
But if you're trying to handle it directly from the database, you'd have to grab the 'active_plugins' value from the wp_options table, unserialize it, remove your plugin from the array and reserialize it and update. You don't necessarily have to use update_option.
You could also look into a command line tool like WP-CLI. With that installed, deactivating a plugin is a simple command from the BASH shell, like wp plugin deactivate plugin-name.