The copies of Follow the Bodies created by its gravestorm ability are created on the stack, so they're not cast and won't cause abilities that trigger when a player casts a spell to trigger.

Gravestorm counts all permanents put into graveyards from the battlefield this turn. It doesn't matter who controlled those permanents, who owns them, or whether or not they were tokens.

You can't sacrifice a Clue to pay multiple costs. For example, you can't sacrifice a Clue token to activate its own ability and also to activate Alquist Proft, Master Sleuth's ability.

Some abilities trigger "whenever you sacrifice a Clue". Those abilities trigger whenever you sacrifice a Clue for any reason, not just to activate a Clue's activated ability.

Clue is an artifact type. Even though it appears on some cards with other permanent types, it's never a creature type, a land type, or anything but an artifact type.

Some spells and abilities that investigate may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't create any Clue tokens.

If an effect refers to a Clue, it means any Clue artifact, not just a Clue artifact token. For example, you can sacrifice Wrench to pay for Alquist Proft, Master Sleuth's activated ability.

The triggered ability that creates the copies can itself be countered by anything that can counter a triggered ability. If it is countered, no copies will be put onto the stack.

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