We have used VBO for changing registration status so if this was not yet currently an option for adding people to the event it feels like not a big stretch, but since automated would be the best route i would be looking down the Drupal Rules approach if civirules can't offer it
You might find this module even better than you expect:
Views Selective Exposed Filters (views_filters_selective)
"Restrict exposed filter values to those present in the result set."
Whenever you're looking at a selection (or subselection), it'll filter the select options to only those options that are in the result set. So you never have the issue of ...
Or any other solution someone found, like limiting the dropdown to used countries only ?
In a Views filter of Country you should be able to select the ~50 that you want listed, and then set the option that says something like "limit to selected items only"
Somebody who is searching for this answer can use the following civcrm extensions.
I have not tested "Activity Calendars" though.
Mostly when working with complex form requirements in Drupal, folk are opting to use webform_civicrm as the flexibility it provides opens up many new options.
For example in your case you could use Webform Conditionals to determine which fields show based on the choice of Membership Type (or type an existing user has if they are logged in or using a ...