Yep, the accounts are linked as friends and family (I was in the same position as @linuss – I only learned about this 6 fpp “method” after buying the party tickets via the regular MDE account). I created the dummy accounts and transferred the party tickets over with no problem (thanks again for your walk-through of how to do it). And you’re right–we otherwise have a room-only reservation and separate tickets (left over from when they were still no-expiration ).
I’ll report back at the 60 day window (which is end of July), and let you know how it goes. I agree that it’s most likely due to the date-specific nature of the party tickets. It just makes me a bit concerned that it is saying I need valid theme park tickets for the dummy accounts (as opposed to showing a message like “Your tickets are only valid for X date; you will be able to make fastpass reservations starting on X date.”)