Not sure if this is helpful, but I live in the US and my mom and sisters live in the UK. I have a Amazon UK account that I use to send them gifts etc. The shipping addresses on the account are theirs and the billing account is my US address. If anything needs to be returned or replaced, it's not an issue. If your mom has issues with the Echo Show down the road, it's probably going to have to come back to the US to be replaced?
I sent my mom an Echo Dot. It came all prepped for Europe, settings, correct plug etc - and a British accent, lol. You might be doing yourself a future favor if you order from Amazon.de.
Having said that, I see the appeal of setting it all up before you send it to her, I spent 1.5 hours on the phone talking my 90 yr old mom through setting up the Echo Dot using her Kindle Fire. Like you, I had limited expectations for how she would use it...listen to books/music. Now I am hooked and want to try and set her up for calls...daunting task, but hopefully worth the efforts!
Good luck!