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No - can be (in extreme) the same student 25 different times.....Very unlikely though.
No - PPL Checkout's don't count.
No - Lapsed PPL's don't count either.
You would normally get the 100 hours long before the 25 signatures.
Be patient though, remember, even though you're being supervised it is still your licence on the line. If you're not happy - don't send them!
HTH