G'day Nicholas
1: My airline (Qantas regional) only has very general guidelines for normal passenger PA's. The first Tech crew P.A. is generally given established in the climb above transition (10,000 ft AGL in Oz). You'll only hear from us on the ground if there is a delay either in boarding or at the holding point.
Having said that, I've heard (and given) PA's using 'departure' and 'taking-off' about the same amount of times.
2: Dash-8 equivalent would be the difference in 16 and 19.5Tonne take-off. About 12kts difference in V1.
3: Depends on the crew. Most say something, but generally what BOAC said (i.e. enough for communication).
DIVOSH!