The first part is very simple. Because you used UA miles you cannot change the name on the booking. Not going to happen. (this is true of all partner award scenarios.) The operating airline can only change a name on a ticket issued by itself.

So, if you want to be 100% sure, you have to cancel and rebook from scratch on whatever airline offers award space. (to be honest, your best option was to not get a new passport until after your trip)

Now, the fact that Maiden is still in the new passport might allow her to fly. There are lots of people with multiple names / surnames who book their tickets in all sorts of different ways. For example I always book my Spanish wife as Name 1, Surname 1, when she has 2 names and 2 surnames - never an issue. However, with Maiden populating a "middle name" field, you might run into difficulty.

The problem, of course, is that you will only find out on the day of your flight. Or even worse, in IST when trying to board your flight to Germany. There's a chance that the TK ticket office at your US departure point can change the surname though, since at T-24 hours the ticket passes to "airport control".