Finally, assuming the AW109 has FADEC, in that case if while in the hover at 100 ft AGL, an engine fails, how long does it take to the live engine to deveop full power and is there a height loss?
Yes, there would be a height loss, how much depending on the ambient density altitude, wind velocity, the all up mass of the aircraft and of course the reaction of the pilot (which would, or should be, to immediately lower the nose, gain airspeed and fly away).
The later A109 series aircraft are quite well off for engine power, compared to most other helicopters of that size but hovering for prolonged periods at 100 ft agl is something that most A109 pilots don't routinely need to do. The manufacturer provides details of "profiles" that afford Class A performance for departure and landing and public transport operations should be carried out with those in mind.