I too expect serious departure from a weighed sum of the enthalpies of fusion. Especially for those two compounds, known and used to make a eutectic.
Yes, published data. It abounds for glycol+water.
Equilibrium with air moisture: I tried for glycerine, and after 15% water was absorbed in the mixture, the mass didn't evolve any more. The process was slow enough (a week for cloth-thin glycerine) that the mass didn't fluctuate with the weather. So for a canister with open tap, you won't see much in years. Only properties very sensitive to water contents, like the conductivity or the viscosity, do change.