Yes, there's a 'fun puzzle' here, but more, much more. I can't go into it deeply here and now, but am keen to see what others think. Carol Kaske may have something to contribute on the subject of renewal and the afterlife. And I believe Terry Krier has published on this sonnet and the preceding one; we had some conversation and correspondence years ago on the subject.
I can't exactly supply 'some strange tidbit of Neoplatonistic trivia/lore,' but as Jack Benny used to say, "I'm thinking!"
Andrew's remarks about connections between the Amoretti and the preoccupations evident in FQ IV-VI are very helpful, not at all crazy. Contexts are always crucial to interpretation.
How is the meaning of the last three words influenced / determined by line 12, 'And in the hevens wryte your glorious name'? What, in heaven's name, does 'in the hevens' mean here? This is 'stellification,' I take it, and what does that mean in Spenser's multiply fictive discourse?
That 'glorious name,' of course, as the preceding sonnet spells out, is triple in its significance: mother, queen, and bride-to-be. LXXIIII ends, 'Ye three Elizabeths for euer liue,/ that three such graces did vnto me giue.' Now, the 'her' with whom Sp. shares a walk upon the strand is only the last of the three, and the waves that twice wash away 'her name' suggest a focus of Spenser's attention on one of the three Elizabeths, but can the content of 'your glorious name' be controlled by the poet, after its publication and his death? What's in a name?
I would add to Andrew's remarks about historical allegory a reminder of my own preoccupation, in print and still today, with the numerous echoes of themes and images from the Garden of Adonis in FQ IV-VI. Back in FQ III vi, Spenser memorialized Sidney as Amintas, 'to whom sweet poets verse hath given endlesse date' (45.9), and he transfigured the myth of Venus' love for the mortal Adonis into a mysterium conjunctionis, the source 'Of all things, that are borne to liue and dye' (30.5). Which brings me round to the end of Andrew's message. Yes indeed, 'they' (and you can choose all the referents you like) cannot be sundered.