Way to check your level stats online

There seems to be no way to unstar a level.

Some anecdotal experiences: 1, 2

There is, however, a way to comment without starring a stage.

1. Play (and possibly clear?) the course
2. Go to Course World
3. Press -
4. Select Played Courses
5. Select View Comments on the course you want Comment in the Miiverse area.

Source

To combine a power-up with a Super Mushroom in a ?-Mark Block:

• Place a power-up somewhere in the level (for example a Fire Flower).
• Drag a Super Mushroom onto the power-up.
• Drag the combined power-ups onto a ?-Mark Block.

Source