First, I do not thing it is splitting hair. It has economical benefits - saves engine life. Should your engine be ready to call it a day, running it at less RPM may just make the difference between make or break. Also, should you have any emergency of any kind, your extra thrust is readily available to you where you look for it - thrust levers, not overhead on bleed switches.
For the second part the answer is YES/NO.
Yes for not using TOGA. When performace is calculated on use of less than full tkof thrust (this thrust is normally still above Max Continious - id est still above Max unlimited) there is no need to select full thrust to achieve the calculated obstacle clearance. Simply, the calculation does not account for re-selecting TOGA, so you do not have to. If you want it, it is all yours to use! But give it a thought beforehand, if your ship is stabilized, climbing steadily, in trim, possibly with AP on and failed engine secured it is a good call. On the other hand if the initial thrust reduction was rather large, selecting TOGA will introduce an extra amount of adverse yaw. During initial moments, this may outweigh the benefits of added thrust both handling-wise (perceived increase in Vmca) and performance-wise (spoilers extended if not handled correctly).
NO for keeping the albeit reduced power indefinitely. Yes, TOGA is limited for 5 mins and 10 with one engine out (or any other number and rule be the book). But unlimited operation is only guaranteed at much lesser power settings CON/MCT - maximum continious/nominal. In the scenario you describe (and 99 day to day ops), the takeoff thrust you selected for departure, is still above MCT hence it may not be used indefinitely. Perhaps the engine run ok for, say 17 minutes, but there is no way we would know, so we must not. The manufacurer's rules are clear as well: operation above MCT is restricted to max 5/10/.... minutes (my wording). Your performance calculations included reduced thrust for the prescribed time only. There is no need to do more.
FD
(the un-real)