Electric hidden unconventional collinear magnetism, in which a perpendicular electric field (E_z) lifts the PT-enforced degeneracy and selectively releases the hidden crystallographic constraints in conventional 2D antiferromagnets to drive emergent magnetic phases, has remained largely unexplored. Here we establish a complete symmetry classification of all 88 collinear spin layer groups (cSLGs) compatible with conventional PT-symmetric antiferromagnets, and show that the E_z drives the system into three distinct classes of unconventional collinear magnetism: type-IV magnetism (Class-I, 23 cSLGs), altermagnetism (Class-II, 34 cSLGs), and fully compensated ferrimagnetism (Class-III, 31 cSLGs). We illustrate these categories using the most common and feasibly exfoliable 2D candidate materials. These include Sr₂CaCr₂P₂O₉ and Sr₂CaCr₂As₂O₉ (Class-I), BaMnGe and MnPS₃ (Class-II), and Sr₃Fe₂O₅Cl₂ and bilayer MnBi₂Te₄ (Class-III). We further demonstrate that the E_z generates distinctly different spin-splitting fingerprints across the three classes, yielding distinct spin-dependent responses: the electric Hall effect in Class-I, spin-polarized transport in Class-II, and the electric reversible magneto-optical Kerr effect in Class-III. Our work not only closes a complete internal cycle from conventional to unconventional collinear magnetism in 2D antiferromagnets, but also establishes a viable all-electrical compensated magnetism platform and control scheme for future spintronic applications.