I am going to put in a good word for northern Germany and the greater region as I am wont to do. ;)
1) relatively easy travel from Boston and must be on Delta or one of its partners
Both KLM and Air France partner with Delta. Easy connections via both Amsterdam and Paris to Hamburg or just go to Amsterdam and take a train.
2) a destination that isn’t overly crowded
Hamburg doesn't get a lot of foreign tourists--mostly Germans. If you aren't there during a festival, you won't feel too cramped.
3) a destination that isn’t too hard to physically navigate as that is difficult for one member of the party who is visually impaired (like the amalfi coast and all the stairs)
All flat around here! Even villages with lots of old town flair are often easy to navigate.
4) a small to medium, quiet ish hotel or resort that is upscale with great food (and must be able to accommodate food allergies)
LOTS of options in and around Hamburg for food, but if you like seafood, you can do well outside of Hamburg--on the Baltic or North Sea, for example. Or stay in a B&B on the south side of the Elbe.
5) if coastal, easy access to the beach or lakes since it will be hot (but not required if we go somewhere less hot)
Like Anna said, Germany has great beaches!
Some more particular recommendations are staying in a small town near Hamburg--I am thinking something like Glückstadt or Lüneburg. For beaches, I cannot say enough things about St. Peter-Ording. I mean, this is hands down one of the coolest and most different beaches you will ever experience. HUGE swaths of "Watt" and few people. There are also beaches directly on the Elbe in the city, though these can be crowded on a hot summer day. Or go to Düne/Helgoland for a few nights. Helgoland can get crowded during the day, but at night it's super chill and really neat, and the beaches are not at all crowded unless you count the seals that might join you on the beach.
For Holocaust, obviously Bergen-Belsen is an easy distance from Hamburg, and Neuengamme is in the city itself and of course the Nikolai church (bombed out ruins), really interesting! For teenagers, Hamburg offers a lot of city life options depending on their ages and interests. But there are some really creative tours and such. Or walk through Sternschanze.
Anyway, that's a few of my thoughts.