You simply need to include in LocalSettings.php the following line:

$wgDefaultUserOptions['visualeditor-enable'] = 0;

This will make VisualEditor be off by default, but enablable for each user under their preferences

Reading https://www.mediawiki.org/wiki/Extension:VisualEditor#Complete_list_of_configuration_options I don't understand what AutoAccountEnable really does. Do you think it can be helpful?

nope, I think what I wrote is what we want, i.e. releasing 0.8 with VisualEditor in "test" mode, i.e. only users who explicitly select it will see it.