It seems there's a lot of misunderstandings about the way that Thetis views her son. For example, in book 18 she states to her sister Nereids:
"Never again will I embrace him striding home through the doors of Peleus' house. And long as I have him with me, still alive, looking into the sunlight, he is racked with anguish. And I, I go to his side- nothing I do can help him, nothing. But go I shall, to see my darling boy, to hear what grief has come to break his heart while he holds back from battle."
I mean that doesn't exactly sound like she hates him does it? On the contrary she is warm and emotional in her feelings for him and he seems to have viewed her in a similar light. Homer tells us that before Achilles set sail to Troy that Thetis even packed a chest with warm clothing for him! Now how tender is that, hummmm?
I don't think that Thetis is alone either in her laments in the poem. After all, most of the women seem to do nothing but mourn or chide their beloved male relatives for fighting. (Briseis, Andromache, Hecuba, and even Helen all have moments of grief and lamentation)I do agree with you that we have to fill in the gaps with our imaginations with the stuff that Homer left out, which can be incredibly frustrating at times. I regret for instance that we don't hear more from Patroclus, who is such an appealing character but it doesn't detract in the end from my enjoyment of the poem!