Technically Yes. You can see everything in both HP areas in one day without EP.
However, if possible, I recommend 2 days. Here's why.
Even in the off season the HP areas become very crowded very quickly. On high attendance days, they can be wall to wall people. Can't see the ground crowded. It can be almost impossible to move around much less look at anything. It's hard to window shop if you can't see the windows through a wall of people. The stores in the HP areas are tiny, more than 3 or 4 people, and they start to fill up.
In addition, the interactive wand spots can get long lines. Sure, you will get a turn, but it isn't as fun to activiate a spell when you have spent the past 5 to 10 minutes watching 5 to 10 people set it off before you. Also, some of the spell locations activate 2 or 3 different ways, so you may want to set the same spell off multiple times, and it's not fun when you feel rushed by a line of people behind you waiting their turn.
So, if the goal is to really take in the details, browse shops, and perform spells, you'll want to do all that as early as possible in the morning. I'd plan one morning for each land then lunch at the HP area restaurants when you get through. That way you are finishing up right when they start to really get crowded. If you are there for the theming, it's worth eating at both the Leaky Cauldron and the Three Broomsticks. Personally, I think both have great food and are a good value for your money as well.
This is what we do on our trips. We schedule one early entry morning for Hogsmeade and one for Diagon Alley if possible or just first thing at opening depending on early entry availability. It's the only way we ever really enjoy these areas as the crowds and lines for wand spots just get so bad as the day goes on.
This would then give you the afternoon to do Hagrids and the Hogwarts Express and not worry about how long the waits are for those rides because you would have already spent a leisurely morning exploring HP. After that, you might be interested in a parade (if running) or show or one of the other attractions in the park.
At US, ET is a slow moving dark ride that wouldn't cause motion sickness (think Peter Pan), and MIB is a fun ride where you get to shoot at animatronic aliens (although some spinning but no screens). Both rides are usually good options for crowded days as MIB has a single rider line that usually moves quickly, and ET is often overlooked tucked out of the way and not a thrill ride.
Also, not sure if you looked at the Mummy. It's in indoor dark coaster. It doesn't go upside down or anything, it might be an option as well. No screens, but a lot of really cool special effects. It's probably Big Thunder Mountain intensity. Sorry, haven't done Gaurdians, so I can't compare to that. It's heavily themed and stops and start as the story progreses, so it's never that bad becuase the runs are so short. It also has a single rider line.
In IOA, you may be interested in the Discover Center where you can see a baby dino hatch and the caves at the back of Camp Jurassic. If there's time and the line isn't too long, you might also enjoy the slow moving Trolley Train (think People Mover) in the Dr. Seuss area. It's motion sickness safe. It's a very underrated attraction that gives you great views looking out over the park. Toon Lagoon is a fun place to explore with great photo opportunities.
This is all just my two cents, though, you can definitely do only the HP areas in the same day if you want. I know two days can by crazy expensive.
No, not if your only goal is HP.
There are really only two main rides, plus Hagrids. If you rope drop and do Gringots and FJ first, they shouldn't have a long wait if any at all. Then, the wand show and spell locations next. Save Hagrids for last as it will be long no matter when you go, so going early would only ensure everything else has a long line by the time you got off.
The only benefit I see to EP in your sitaution would be if you want to ride HP rides over and over or just do not want to wait at all ever (excluding Hagrids) becuase even without EP, there is plenty of time to see everything even if you have to wait a little.
BAD
Honestly, I can ride everything at Disney and almost nothing makes me sick, but at Universal, it's the opposite. Almost everything makes me sick with FJ being one of the worst offenders. I have to get motion sickness patches from my Dr before our trips. Depending on how bad your husband's motion sickness is Gringotts might get to him as well. It's not as bad as FJ, but it is a mix of screens and real motion and does effect some people.
A few other general tips for touring HP.
1. Look up, there's great details in the ceilings and around the tops of buildings that people never notice.
2. Take time to really explore, look in windows, listen at doors, tons of little details and Easter eggs all over.
3. You can exchange your muggle money with the goblins for wizarding money. You can also just stop by and talk with them as well.
4. There are 4 kinds of butterbear (regular, frozen, hot, and ice cream), and some people make butter beer floats as well.
5. There are also other wizarding speciality drinks you can try as well that often go overlooked.
6. Check out the different wizarding candy and bakery items.
7. Don't miss Knockturn Alley. There is even a dark arts store in there.
8. You can send mail back home by owl post, bring a stamp and postcard or buy one there.
9. Take time to eat in one of the themed restaurants or the ice cream shop.
10. Ride the Hogwarts Express both ways. To Hogsmeade, have one person go ahead in line when you get to the plexiglass, and the other who hangs back will be able to watch the first person dissappear between platforms 9 3/4. Just be curteous of peolple in line behind you and don't block the path.
11. Catch the different stage shows in both areas.
12. If you care about the storylines, each main ride has a series of little preshows in each room to watch as you go through. Most people just pass on by and don't listen to these, but they are really good and set the tone for the ride, so you may want to stop and let others pass you in line so you can listen to them fully. Sometimes you enter in the middle of one or as it is ending, so you have to wait for it to start up again.
Whatver you decide, you'll have a great time. A first visit to HP World is always a magical awe inspirig experience.
