I think the two monks (like Ajahn Thanissaro before them) are quite correct in their reading of Mettasutta’s “mother and her only child” simile. The grammar does indeed require that one’s unlimitedly mettāful state of mind (not “all beings”) be taken as corresponding to “the only child”.
As for their claim that there’s no love in lovingkindness, this strikes me as only trivially true. Ven. Ñāṇamoli has apparently decided from the outset that he’s going to limit the meaning of “love” to those senses that correspond to the Pali term pema (i.e., affection involving attachment). If you do that, then it does indeed follow that “there’s no love in lovingkindness”, but only because you’ve made it so by arbitrarily limiting the semantic range of “love” and disregarding all the reported senses of the word that don’t correspond to pema.