Ultrasmall-angle X-ray scattering is employed to probe the local and long wavelength collective structure of ternary mixtures of silica nanoparticles suspended in concentrated solutions of polyethyleneglycol in the good solvent ethanol. In the dilute polymer limit, these suspensions are stabilized via electrostatic repulsions, at intermediate polymer concentrations the nanoparticles aggregate, while in the polymer melt limit they again become homogeneous due to the formation of thermodynamically stable discrete adsorbed polymer layers on the nanoparticles. Solvent dilution of the polymer melt is found to modify the nanoparticle concentration fluctuations on all length scales in a manner that can be understood as a monotonic reduction of the favorable polymer particle interfacial cohesive attraction. The measurements are quantitatively compared with predictions of the two-component Polymer Reference Interaction Site Model theory modified to account in an implicit manner for solvent addition via an effective contact strength of interfacial attraction, epsilon(pc). Good agreement between theory and experiment is found without adjustable parameters based on the idea that epsilon(pc) linearly decreases with solvent volume fraction. The joint experimental-theoretical work suggests a new mechanism for restabilization of aggregated particle suspensions at high polymer concentration based on the thermodynamically controlled interface cohesion of weakly adsorbing polymers that contrasts with the classic kinetic repulsive barrier mechanism under nonadsorbing depletion attraction conditions.