Most run-ups to 60% are to blow ice accumulations off vanes, etc. behind the fan that might have formed during taxi conditions where the inlet is depressed due to suckingt the air. Some run-ups also are designd to shed ice accumulated on the aft side of the fan blades again due to air being sucked through these blades at idle.
But on the subject of blade twisting with RPM. Yes they do twist-untwist and it is significant for the narrow chord blades that have those noisy shrouds or clappers between each pair of blades.
Yes the incidence angle is very important (at high RPM) and so is the ability to handle vibratory stresses and the ability to replace these blades on-wing. So the designer designs the blade to fit loosely in its disk slot and to accomodate a twisting action as it inceases in speed until all those shrouds. snubbers, clappers etc. between the blades twist into each other and form a soilid ring and freeze anymore twisting at the optimum incidence angle for high power.
It's always interesting to find out the root technical basis behind some of the recommended procedures published for the equipment one flies
