Cooling: There is no objection against any means to cool engine and transmission.
O/D: Of course you have that switch on side of the shifter to disable overdrive: it lights up the HOLD light on the dashboard ; it also has the effect of starting in 2nd..but switching to normal for accelerating then HOLD once in 3rd is OK. I would do that: in fact I do that every time I'm about to face a moderately steep hill: downshift to 3rd: the little engine revs shoothly but has no torque below 4000...
PWR/ECO: THe power/economy button is nothing but a gadget that simply shifts transmission at a slightly different rpm so no surprise it does not make any difference at highway speed.