Yay your trip is so close!
That's great that everything is paid-off! What an awesome feeling- now there's nothing left to worry about- just fun to plan for!
We preordered PP+ last time and we loved having it! We stopped for just about every photographer too
and ended up with over 1,000 pictures. Definitely got our money's worth!
We did both Universal parks in one day on our trip in January. I think it was a really good idea not to take the kids with you. My DBF's 2 year old niece was with us when we went and there really wasn't a whole lot for her to do, other than the designated kid areas. She spent 95% of the day sitting in her stroller waiting for us to ride things. Even the shows we stopped for weren't great for her. There was fire and loud noises and she was scared most of the time.
That said, you should have no problems doing everything that you want to do in one day. I don't know what the crowd levels were when we were there but the only things we waited on line for were in Harry Potter word because we didn't head there first because I was the only Harry Potter fan in the group.
Even with the baby we did Hulk, Storm Force Acceletron (basically fancy teacups that I wish we hadn't done), Spiderman, Dr Dooms FreeFall, Jurassic Park River Adventure, Harry Potter world, the Eighth Voyage of Sinbad Stunt Show and Poseidon's Fury (which is described on their website as a special effects spectacular and DBF's brother told us it was so cool we couldn't skip it, but it was pretty long with a lot of standing around and waiting and it was alright but I didn't see anything too amazing about it so I would say you could skip it). Then we rode a few things in Seussland and spent some time in the candy store and the baby met some characters. For just the two of you, I think you would probably enjoy walking through here because it's amazingly well-done but not spend any real time here and we left IoA before 3.
We had eaten a really early lunch in IoA so when we left, we stopped for food/snacks on Citywalk on the way over to Universal Studios. I'd say we took a break sitting in the air-conditioning for a good 45 minutes to an hour because the baby fell asleep and we were all pretty worn out.
So then we headed over to Universal Studios and did Shrek 4-D, Hollywood Rip Ride Rockit x 2, Revenge of the Mummy (twice due to child swap), Twister and Men in Black x2 before it was time to see the nighttime show and the park was closed. We didn't make it to everything in USO but DBF and I had just been there and the other adults we were with cared more about IoA. Plus, we stopped a lot with the baby to rest and change her, etc.
Sorry, that ended up being a lot longer than I had planned. I wanted to go back through everything we did to make the point that you should most definitely be able to do all of your must-dos and more. I definitely recommend hitting Harry Potter world first because that's pretty much the only place we hit really long lines.
We took the Mears shuttle on our first trip and we found it great. The only thing I would say is, if you want to get there for park opening, just make sure you schedule an early enough shuttle because they can stop for other families on the way. We took it twice on our first trip and the first day we had the shuttle to ourselves and the second day we stopped at 2 other hotels after ours and barely made it there for park opening. As far as coming back, I had already scheduled our pick-up time ahead of time and then when we were in the park, we realized we would be done earlier than we had thought so I called them and was able to move our pick-up time to 90 minutes after I called. So you should definitely have enough flexibility! I would probably say call when you're part of the way through with dinner so that you're not rushing to get through your meal to make your pick-up time and if anything, you can always spend some time exploring Citywalk after the meal. Plus, it's a little bit of a hike to the area where the shuttles come!
Sorry for the essay, I hope some of that helps!