The copies of Friarball created by its coststorm ability are put directly onto the stack. They aren’t cast or played.

Spells cast from zones other than your hand and spells that were countered or otherwise failed to resolve are still counted by the coststorm ability.

A copy of a spell can be countered like any other spell, but it must be countered individually. Countering a spell with coststorm won’t affect the copies.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.