Do you need constant internet access for D-Ticket?

Just about every German train has wifi available so you shouldn't have any problem showing your QR code. You could take a screenshot "just in case" but sometimes QR codes do not work when scanned from a screenshot.

I really wouldn't worry about it - as I said, the wifi is almost always available.

So it if I do the math even remotely close - the d-ticket is a great deal.
We would use it:
train from FRA airport to Boppard
train from Nuremburg to Rothenburg
train from Rothenburg to Nuremburg
While in Nuremburg - the tram to the zoo - and maybe a short trip to two on the U-bahn/S-bahn
Train from Nuremburg to Munich (if we skip the ICE train)
Then Munich HBF to Isartor on the S-bahn
Finally a couple of U-Bahn trips before the calendar turns to August 1 where we will then buy day passes or individual tickets for the last three days of our trip.

Am I missing something or is this a no-brainer?

Just need to
1. Find the app that takes US credit cards - Fahrplanner or MVV app I have read
2. Buy after July 1 and cancel prior to July 10th (trip is July 20-Aug 3)
3. Each of us need to have our own version of the app.

No, you don't need internet access the entire time. It will be loaded onto your app that you bought it with. (I used MVV app). We were able to pull it up from the app without difficulty. I did also take a screenshot as a back up but I never needed to use it. The D ticket is a great money saver and so convenient. Have fun!

The MVV app is very user friendly and entirely in English. I was able to buy my ticket the month before and I cancelled the subscription about 5-10 minutes after buying the ticket and having it downloaded into the app on my phone.

Note: the Subscription is by calendar month. Those first three days of August would not be covered on your July pass. You will need to see if it wouldn't be more cost effective to have D ticket for July and then indiv. tickets for August.

Not wanting to start a new thread - hopefully someone can answer.
When in Munich and we want to use the D-ticket on the S or U-bahn - Is there a turnstile to scan to get through? I wonder what happens if the screen shot doesn't work in this situation? I thought I read somewhere there isn't actual turnstiles (like in Paris or New York for example) that you have to scan to get to open?

No turnstiles on the S or U Bahn. You just get on and off. If a fare inspector were to ask for your ticket you would show your D ticket. I was never asked for my ticket. I was asked to show it on the regular train where it was scanned. I used it often on trams and buses and it was not scanned. I just showed my screen to the operator as I walked past.

In Garmisch, I had the same bus driver on multiple days. He just waved me on without even looking after seeing it a couple of times.

Just about every German train has wifi available so you shouldn't have any problem showing your QR code.

Please note that this is true only for long distance trains, not for (the majority of) local trains. But since the DT

will be loaded onto your app that you bought it with

that's completely neglegible.

There is no "paper" equivalent that I could just print off a couple of copies of and then show if asked?

No, that was only for a transitional period.

QR codes work from a screen shot. It's the same thing. It is the information encoded in the image, and it is the same reason that printed out QR codes also work.

and it is the same reason that printed out QR codes also work.

Since December 31, a paper printout of the DT is no longer a legal proof of ticket ownership. I would not rely on it.