Traveling with elderly parents. Itinerary help

I think the pace you have looks good, and I would probably leave it at that. Munich would be the place I would consider adding if you decide you have extra time. I would think about 2 things before adding it. One is how easy or difficult it would be to have a car in the areas of Munich you would want to see. Public transportation there is great, but using it would depend on if that would work for your parents. The other is that seeing Munich would require another overnight stay in a different location. It looks like the way it is set up, you could be staying overnight in just three locations if you stay in the Frankfurt area for the first three nights. Three locations in that amount of time is a good pace and about how many I would choose.

"Anything I missed that is a must see?"

You should not be concerned about this, IMO, but rather whether the "sees" that you pencil in will be doable and enjoyable for YOUR family. It would probably not be my objective to visit Rothenburg, no matter its name familiarity or how many tour buses go there - too touristy and too crowded - when nearby there are some enormously lovely, equally charming, and more peaceful places they might visit instead.

Iphofen - NE of Rothenburg - photos
Büdingen - NE of Frankfurt - photos
Ochsenfurt - N of Rothenburg - photos

Car traffic within Rothenburg is severely restricted as well - which may make for a long day on foot for your folks:
R'burg traffic controls

Neuschwanstein (not a castle but a 19th-century palace) is likely to be problematic for the elderly in a similar way. Long uphill walk, tour with lots of steps, throngs of tourists. You may find it easier to visit nearby Linderhof.

There are hundreds of things you have missed in Bavaria. But what would interest your family the most? You can share your interests and get suggestions here and/or do some more independent research.

My parents request was to see a castle, scenery and places that aren't to "modern". As mom puts it "storybook" Germany. The only places on the itinerary that are an absolute must are Konigshofen and Glattfelden.

My parents can get around pretty good as long as they can stop and rest periodically. I'm definitely open to suggestions since this is a once in a life time trip for them.

One of my favorite castles (and Rick Steves' favorite castle in all of Europe) is Burg Eltz - unlike N'stein, it's a real castle with a history of 800+ years, and unlike most castles on the Rhine, it was never destroyed. If you'll be visiting the Rhine, you're almost there anyway, so go there and have a tour of the place:

Burg Eltz

A few miles from this storybook castle is the storybook town of Cochem (on the Mosel River) which has its own castle and a fabulous old-world town center.

View of Cochem
Cochem's market square

I would suggest 2 nights in a Rhine village (Boppard or St. Goar.) Visit the village of Bacharach and/or Oberwesel to the south your first day.

Oberwesel
Bacharach

Drive to Burg Eltz and Cochem the 2nd day and return to your Rhine Village for a 2nd night.

"My parents request was to see a castle, scenery and places that aren't to "modern"."

If you're going to be in Darmstadt anyway (nice town, but mostly modern except for some preserved buildings in the center and the well-known Jungenstil buildings), drive south of the city along route B3 (called the Bergstraße, or "mountain road"). You'll drive through several well-preserved old towns, including Bensheim, Ladenburg, Heppenheim, Zwingenberg, Weinheim, although you'll have to get out of the car and explore on foot to see the historical cores. There's also a chain of castles that run along the mountains overlooking many of these towns.

And a note on genealogy. Make arrangements prior to arriving. Many civic records were documented in a form of script that today even many native German speakers can not decipher. Also, Darmstadt got hit particularly hard during the war. The records you seek may no longer exist.

Thanks Tom,
As far as the genealogy part goes, I've been able to get many of the documents I want from other sources so while I'll probably stop by and see if I can find anything new, it's really more about visiting the places. I'm certain towns like Darmstadt will most likely look nothing like the time period of our ancestors (pre 1834) while other towns where ancestors came over later will hopefully have at least a few buildings left from the time period. I know Konigshofen where my grandmother was from was almost completely destroyed just a couple days before the end of WWII as the Germans made their way back to Berlin.

How would the Darmstadt area be as a "home base" for a few days and do day trips to the areas suggested. Most seem to be within a couple hours drive a Darmstadt.