I always use Undercovertourist when buying Universal tickets and I have never had an issue with them. If you're worried about using the tickets from UT to scan into the park, you can go to guest services and exchange the printed UT tickets for a paper ticket (and a separate paper Express pass). I did this in May and it was super easy. Make sure to bring the credit card you used to pay for the tickets on UT in case they ask for it.
I've never had an issue but of course with disney you load it into the app and at universal once I scan in I go to customer service and upgrade a seasonal pass
You can load it into the app and scan from your phone. Or as PP said, you can exchange it for one of the official smaller UO tix. The guest services wait times at the front entrance can be very long. I recommend exchanging it at your hotel ticket desk if you are staying onsite. If not, use the UT ticket to get in the first time and exchange it at the inside guest services desk which usually takes less time.