'If takeoff power is for takeoff purposes only why do the ‘hover ceiling’ graphs specifically state ‘takeoff power’?'
The manufacturers rightly consider you will have to put the aircraft in hover before taking off.
'Why do we not have hover performance graphs to use for the maximum continuous power rating?'
Some aircraft have different graphs for hover at TO power and at MCP. For the 365 N2 the difference in MTOW is 720 kgs at 40 C.
ATN