Only some precisions.
The apartment is 25 square meters. ( like is written in the list)
The two closer metro station (two diferent lines) are exactly 500 meters from the apartment. But you can visit 80% of city by foot. Of course are lot of buses too.
I don't understand why is dificult to come back home. In this zone of Barcelona the streets are parallels or perpendicular, like a grid, so it's very easy to orientate. (and in the apartment there is a map)
The stair is narrow (but only 20 steps) but not dangerous. Sori is 27 years old, I'm close to 60's and never I have any problem in the stairs (I usually help the guests with their baggage without any problem).
Finally about the hot water. The plomber recomend us a 30 lt tank for 2 people. I order to install a 50 lt tank. I think is enought for 3 people so for 2 is more than enough to take a shower. And with my wife (and also sometimes with our daughter) we have taken all the shower (normal shower) in a row.
I have a look about shower draining.
It was a pleasure to host you