I see the musical being a seperate episode. Only because the Axew quest might take a little while, and with the sports dome stuff the battle in the latter part of the episode would suffer.
That's why it makes no sense for me, why the musical will be in BW053.
Sure, I'll agree the sports dome, with tennis, and all that will be in BW053, but the musical looks like a completely separate episode.
I mean if a random convention "Hearthome Convention" got it's own random episode, I can't see why muscials wouldn't, considering it should get its own episode.
If it is in BW053, then so be it, it might give Oshawott a chance to evolve prior to Clay, maybe even sooner then that.
But the musical looks like an entirely different episode to the Subway Master episode.
Edit: Although I disagree with the Axew Subplot taking too long, it shouldn't take more than a minute.