There is some canonical basis for Yavanna's interest in Middle-earth: in the Years of the Trees, "Yavanna was unwilling to utterly forsake the Outer lands for all things that grow are dear to her and she mourned for the works that she had begun there and at times she would come and heal the hurts of Melkor." So I think it quite plausible that she would have kept this up in the First Age. Although, as you say, sneakily.