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A law becomes useless if you keep tweaking it this way and that.
Literal
Law [quotative-というもの] [topic-は] this-and-that adjust-[excessively-すぎる]-[if-と] any-of role [into-にも] not-become
Several meaty pieces. ~というものは is a definitional/characterizing topic marker — 'a thing called X', 'what we mean by X'. あれこれと is 'this and that, in various ways'. 手加減 is 'adjustment by degree, going easy' — 手加減する = to adjust the intensity of something. ~すぎる ('do too much') is the excess suffix. 何の役にも立たない is a stock phrase for 'is of no use whatsoever' (with emphatic も).