Questions -
They both mean the same. It is the amount of temperature rise required to get the incoming water to the required outgoing temperature. Here we use an average incoming temperature of 10°C so if you required an outgoing of 60°C it would be classed as 50°C ΔT.
Basically anywhere, outside the building. If the position of the heaters complies with the positioning of a concentric boiler terminal as per BS5440 then it is fine.
No, all Rinnai Water Heaters will protect themselves down as low as -15°C providing the electrical supply is left ON. They will NOT however protect any external pipe work against frost damage this you have to take care of. If you fit an external pipe cover box this will assist in the insulation of the pipes.