I checked in at DB's Reisezentrum at the main station in Mainz this morning and was referred to the main official there. He told me I could not buy one in Mainz (which I knew and wasn't planning to do) and he confirmed that you can walk into the Frankfurt Airport or Frankfurt Hbf station Reisezentrum and purchase the rail passes that appear in the chart on the PDF file linked to at the DB German Rail page. I assume that other main stations which have previously been listed on the DB site as Eurail aide stations can also sell you the pass in the event you aren't flying into Frankfurt. Here is a link that gets you to that list, per the Eurail site, in case you need it:
https://www.eurail.com/en/plan-your-trip/trip-ideas/trains-europe/useful-train-information/eurail-aid-offices
I asked him for a price on the 5-days-in-30 twin passes, and he quoted me 478 Euros (2nd class ) and 638 Euros (first class), which of course covers both of you - the same prices that appear on the chart mentioned previously.
I have only bought 2 of my several German Rail Passes in person. But I can tell you THAT is how I prefer to buy the pass. Instead of the unforgiving internet, there's a person there who knows what they're doing and can ask you the right questions, and you can get valuable advice on how to use the pass and how to fill it out properly, and if you do enter a date incorrectly (as I once did) or something similar, you'll know what to do and NOT to do. And you can ask all the questions you like as well.
I like the paper one. I don't scrapbook, but it's nice to have the hard copy to look at years later. I have a small cloth belt-pouch which I sewed McGyver-style using a pair of worn cargo pants in 1974 and which I used to carry my rail pass and passport in on my first rail pass trip. It was a laminated card back then. But I've also hung onto my paper ones. And it's a blast to just pull them out every couple of years and run through the memories that those passes bring up.
As far as adding the pass to the Navigator app goes... I don't really understand that concept since there are no specific itineraries on your German Rail pass. I am not app-literate - hardly bother with my cell phone when I travel in Germany. But you can of course handle any prospective itinerary using the app, right?