<p>A double-helical (DH) order, characterized by alternatingly nested single helices, represents a unique noncollinear spin texture in antiferromagnets, emerging from unevenly frustrated exchange interactions. Leveraging a frustrated planar-anisotropic spin model, we investigate the intricate magnetic phases of the DH antiferromagnet YMn₆Sn₆, which exhibits a high Néel temperature (<i>T</i>_N = 345 K). Despite a weak magnetocrystalline anisotropy constant (∼0.3% of the interlayer exchange interaction), it significantly influences the evolution of field-driven phases. Visualizations of spin textures reveal that an in-plane magnetic field transforms the DH ground state into a distorted conical spiral (DCS) state, challenging previous simplistic depictions of conical spirals. The rapid suppression of the <i>c</i>-axis spin component plays a key role in the emergence of a fan-like intermediate state before saturation. Notably, this newly identified DCS phase coincides with a near-room-temperature (<i>T</i> = 250 K) sign-tunable magnetoresistance effect, exhibiting a ∼40% variation. This behavior is linked to multiple interlayer hopping amplitudes, suggesting a deeper interplay between spin texture and electronic transport. These findings highlight the critical role of anisotropy in DH antiferromagnets and offer insights into noncollinear antiferromagnet-based spintronics and transport phenomena.</p><p></p>