The spin-1/2 kagome Heisenberg antiferromagnet (AFM) provides a paradigmatic platform for realizing quantum spin-liquid (QSL) and magnetization-plateau states. Experimental progress, however, has been hindered by structural disorder, whose effects on these quantum states remain elusive. A key experimental challenge is therefore to connect intrinsic and disorder-dominated regimes through controlled disorder and identify emergent degrees of freedom associated with disorder. Here we introduce controlled point defects through electron irradiation into the spin-1/2 kagome AFM YCu3(OH)6.5Br2.5 (YCOB) and track the resulting evolution of its low-energy magnetic states. High-field magnetic torque measurements reveal that the 1/9 and 1/3 magnetization plateaus are strongly suppressed by only ~0.05% irradiation-induced defects, demonstrating that these delicate quantum many-body states are extraordinarily fragile against dilute disorder. Moreover, the irradiation induces a sign reversal of the magnetic torque only at low temperatures and low fields, indicating the emergence of correlated defect-induced moments rather than isolated free spins. Remarkably, the characteristic scaling of the magnetic specific heat, absent in pristine YCOB, progressively emerges with increasing disorder. These findings establish dilute disorder as an effective tuning parameter that drives the kagome AFM from intrinsic quantum states toward a disorder-dominated regime. The contrasting responses further identify pristine YCOB as an exceptionally clean kagome AFM for accessing intrinsic quantum states.