Tom775257,
The run-up you mentioned was probably for icing considerations. With our RR engines we must run-up the engines to 60% N1 for 10 seconds whenever an icing condition at 0C OAT and below. If they did run for 30 seconds I can only assume that they were receiving there take-off clearance at that moment and chose to leave the power up rather than pull it back and to idle and forward again seconds later. No it should not have anything to do with the engine failure as if they were concerned about an engine problem they would not have departed especially with a twin!
Clarrie, I don't totally understand your point but take-off's are designed to use as much of the runway as practical and therefore if light we would use a reduced thrust take-off.