I had said this above, but I'll reiterate:
I bought a 2-day parkhopper from UndercoverTourist at about 8:50am this morning. I got the e-ticket in my email less than a minute later. I opened my Disneyland app, logged into my account, and scanned the barcode linking my ticket to my account.
When the time finally came at 2:20pm to reserve my days, I was directed to the page and my ticket was already linked there. Also, about when I had 2 minutes left on my countdown I opened up another tab and made sure I was logged into my account so that there would be no issues there. Your account will auto-logout after a certain amount of time.
The transaction ran smoothly. There is no need to fear purchasing your tix from UT and linking them to your account via the app.