Provide an easy way of disabling the functionality for dev environments

1. I set the default variable value to TRUE so it shouldn't affect existing users in any way. There's no need for a hook to update it, it will just bypass my check until you manually set it to be FALSE in the admin settings.

2. I considered writing "This is useful for development environments" in the description text for the setting, but as far as I know this is fairly common practice and those who need it know how to use it, I guess. Examples of other modules I use which provide the same type of setting are:
https://www.drupal.org/project/advagg
https://www.drupal.org/project/reroute_email

For the advagg module the setting is presented as:

Label: Enable advanced aggregation
Description: Uncheck this box to completely disable AdvAgg functionality.

For the reroute_email module it's presented as:

Label: Enable rerouting
Description: Check this box if you want to enable email rerouting. Uncheck to disable rerouting.

And here's how we would use them in settings.php on a dev site:

$conf['reroute_email_enable'] = TRUE;
$conf['advagg_enabled'] = FALSE;
$conf['livechat_enabled'] = FALSE;

The reason being of course that we don't track settings.php in our git repositories and that way we can safely sync the database from a live site to a dev site and not have to worry about manually setting all the dev environment variables every time we do so.

3. I haven't used that module, but this is a variable just like any other so I don't see why not.