Take a look at Google Maps. It appears that much of the walk will be along the quay. I'm doubtful the first part of the walk will be very interesting. However, it wouldn't surprise me if you'd need to do that first part anyway, in order to reach a taxi.
Google Maps may allow you to see pictures of the walk you'd need to take. I don't do that very often, so I may not have this exactly right, but if you fiddle around with it, you'll be able to figure it out:
- Go to maps.google.com. Enter Drassanes or Paral-lel in the Search box and click on the option with the little train icon, which identifies the Metro station.
- Click Directions and enter Moll Adossat as your starting point. Be sure the icon of the walking man is highlighted at the top left. You'll see the walk plotted. (If you wanted public-transit routing, you'd want to click on the train icon instead.)
- Click on the small square at lower left with an aerial photo; the label may say "Layers" or "Satellite". That switches you to an aerial view of the walk.
- Click on the "+" at lower right to zoom in a bit. At bottom right is a little yellow man. Drag him onto the walking route and you should get a pedestrian's (not aerial) view of the surroundings. I wasn't able to get anything for the first part of the walk, but it worked after the first left turn on the walking route.
- The little spinner above the "+" sets the direction of the walk, then you can click on the "^" symbol (could be pointing in any direction) on the photo to move in the indicated direction. This sounds awkward, and it is --at least for me. I don't use this feature of Google Maps very often, so I'm not good at it. But it is handy for checking out a neighborhood if you have enough patience.
Buying Metro tickets will be easy. The vending machines will have an English-language option (look for the British flag if you don't see an "English" button). I don't remember hearing about issues with using US credit cards. If you have euro currency, that should be usable as well. There may also be a staffed ticket booth. Note: Some subway stations now allow you to ride by just tapping your credit card on the entrance gate, so you don't have to go through the step of buying a ticket. I don't know whether Barcelona allows this. You should be able to find that info via Google. I haven't used public transit in Barcelona since 2016 and am not up on the current procedure.
You can use Google Maps as described above to plot the path from La Sagrada Familia to Parc Guell and from Parc Guell to Casa Batllo. The short answer is that Parc Guell isn't really within walking distance when you have just one day in the city. It's uphill on the way to the park; on the way back to the center of the city, it's a pleasant and interesting downhill walk, but not really short enough to be practical for you. Even taking the Metro may not be advisable, because the Metro station is a lot lower than the park. There are buses (Google Maps will provide routing info if you click on the train icon at the upper left) that get you closer to the park than the Metro does, but in your position I'd take a taxi from the church. Barcelona taxis are not terribly expensive, and your time is valuable.
Casa Batllo is walkable from La Sagrada Familia at just over a mile, if you decide to visit Casa Batllo before Parc Guell. The Eixample is an attractive neighborhood, and you might bump into other Modernista buildings along the way in addition to the obvious ones in the same block as Casa Batllo. Google Maps will give you public-transit options that would save a few minutes if you don't get lost on the way to the Metro station or bus stop. (I'd walk.) I can't guarantee there will be a ticket-vending machine at a bus stop, so it would be smart to plan your transit use ahead of time and buy more than one ticket at the first Metro station you encounter.
Out of space. Continued in next post.