I will be in Croatia-- Dubrovnik, Split, side trips to Mostar, Montenegro, possibly Korcula. Will there already be throngs of tourists from June 22nd-June 29th? Or do the majority begin to come in July?

We will be in Slovenia the first week in July-- that should be fine.

Even though most of Europe won't begin their school summer holiday until July (some parts of the Netherlands start the last week of June), that won't necessarily stop childless couples from going earlier (or later). Plus Americans will already be there.

Best to go expecting throngs of tourists that way you can be pleasantly surprised if it's not as bad as you planned for.

Try to check cruise schedules for Dubrovnik and avoid days when the big cruise ships are in town. They can completely clog the inner city so that is literally impossible to move and your days are ruined. There are websites that show what ships dock where when. Find them and try to plan your time in Dubrovnik accordingly--if that's possible. Otherwise a fantastic city.

the key issue is the terrible cruise liners, if they are in port the place becomes unbearable

This is the site for the Dubrovnik cruise ship schedules (although it's down right now): http://www.portdubrovnik.hr/girica1.php