Cubic terrain... Is this minecraft?

It's possible but not mentioned in the pixiv post. The artist also hasn't done any previous minecraft works. Best to leave it out.

where's the "most unlikely bands" pool?

Of course, no present band would be complete without a tambourine.

That's not exactly a keyboard; it's a melodica . Adding tag, but leaving "keyboard (instrument)" tag just in case it technically counts as both.