This study presents a comprehensive density functional theory investigation of the structural, energetic, and magnetic properties of Pt147-nNin (n = 0,1,13,43,55,147) nanoalloys across the icosahedron, decahedron, and cuboctahedron motifs, supplemented by analyses of the L10 and L11 ordered phases. Systematic evaluation of mixing energies, bond-length distributions, and shell-resolved magnetic moments reveals that physical properties depend sensitively on chemical composition and geometric motif. A pronounced stability window is identified at the Pt104Ni43 composition, where the mixing energy reaches a minimum and bond-length contraction is maximized. Ni atoms provide the dominant contribution to the total magnetization across all motifs, with the largest local magnetic moments occurring at intermediate Ni contents (43 Ni atoms), coinciding with the minimum mixing energy. Comparisons with the L10 and L11 ordered phases highlight a potential role of chemical ordering in magnetic enhancement, but a definitive separation from composition effects is not possible within the present dataset. Collectively, these results reveal consistent correlations between composition, structural ordering, motif selection, and magnetic polarization, providing guidance for the rational design of Pt-Ni nanoalloys with tunable magnetic properties.