Empty Meta Cleanup for WP Job Manager

To cleanup existing empty meta entries in the database, you must be have either the WP Job Manager Field Editor 1.12.2+ or Search and Filtering for WP Job Manager 1.1.9+ plugin installed and activated. The code that handles that is included in those plugins, and is not something included with this plugin.

It depends. Does your theme or another plugin add custom fields outside the default WP Job Manager (or associated) fields? Have you changed any of the fields to optional? Are you using the WP Job Manager Field Editor or Search and Filtering for WP Job Manager addons? If you answered yes to any of these, then yes.

Currently the method used to handle this is by looping through all the fields after the core plugin saves the listing, checking if those values were actually submitted when the listing was saved, and if they are not, it deletes the meta from the database. In an upcoming release the core plugins will be short-circuited when they attempt to save the meta, but for now we just delete the empty meta value after it's added to the database.

Yes you can, please check GitHub project for associated filters that can be used (a UI will be added in a later release for this)

I strongly recommend also using the "Index WP MySQL for Speed" plugin to add indexes to your database: https://wordpress.org/plugins/index-wp-mysql-for-speed/