Corrected entry: When Peter first has a piano in his bar, it's an upright model commonly seen in bars. The next night, it's a full sized Grand Piano. Given the width of the stairs, the size of the doorways, it seems unlikely to have been put down there without a considerable amount of effort.
Corrected entry: When Peter builds the bar downstairs, and Lois sings for the first time the wall that lines the stairs is solid. When she has a stage and curtain backdrop, suddenly there is a door where Stewie is in the jolly jumper.
Correction: This door could have been added when the stage was built. There is no time frame given from the building of the bar to this point.
Corrected entry: When Stewie's chatting up 'Misti', the equation he writes on his napkin changes between shots. (00:11:40)
Correction: The whole area was transformed, a stage, extra lighting, etc. So, yes, a considerable amount of effort was done. Lois was excited to perform and conceivably hired a bunch of professionals to fix the area up (who are more than capable of bringing a piano down stairs and setting it up).