Much food for thought there.
Three miscellaneous points, I'd pick up:
(1) Does Mopsus in the Eclogues strengthen Dix's argument as to the wider significance of the Grynean grove (i.e. because that had a Mopsus and a Calchas)?
(2) Arcadia and Vergil v Arcadia and Gallus
(3) Callimachean poetics in Eclogue 6
(1) Does Mopsus in the Eclogues strengthen Dix's argument as to the wider significance of the Grynean grove (i.e. because that had a Mopsus and a Calchas)?
(1) Dix didn't deploy this point but it does provide incidental support the idea that the Grynean grove had indeed become significant in Latin poetry. The name Mopsus doesn't appear in bucolic before Vergil, so far as we know.
But there is a possible alternative explanation for Mopsus - Menalcas at Ecl. 5.10f asks him to sing 'aut Phyllidis ignis / aut Alconis habes laudes aut iurgia Codri'. There is a compelling argument that Codrus signifies Messalla - Nisbet (2002) is Robin Nisbet "A Wine-Jar for Messalla: Hor. Carm. 3.21" in "Traditions and Contexts in the Poetry of Horace", ed. Woodman and Feeney, Cambridge 2002 (cf. ('sober scholars have approved the the idea that 'Codrus' in [a fragment of Valgius] is none other than Valgius' patron [Messalla].' Hollis (2007)), part of the argument being that Mesalla, writing bucolic poetry in Greek perhaps contemporaneously with or earlier than Verg., seems to have advertised his poetry as savouring of Attic style - 'carmina cum lingua tum sale Cecropia' ([Verg.] Cat. 9.14), Codrus on Nisbet's argument is a reference to the legendary king of Athens *who dressed up as a shepherd*. Perhaps, Mopsus has related significance - i.e. relating to 'Mopsopus', another legendary king of Athens - cf. in particular, Tib. 1.7: Sic venias hodierne: tibi dem turis honores / Liba et Mopsopio dulcia melle feram.' addressed to Messalla (cf. Call. Fr. 709 Pfeiffer (Mopsopia = Attica) and, indeed, Euphorion's poem entitled 'Mopsopia'): i.e. one might say that Mopsus, himself, is, perhaps among other things, a mask for Messalla. Perhaps, 'Alconis' in 'Alconis ... laudes' could be intended to gloss 'Valerius' by reference to Greek ἀλκή?
(2) Arcadia and Vergil v Arcadia and Gallus
(2) We know since e.g. Schmidt, Kennedy and Jenkyns that the landscape of the Eclogues generally is not Arcadia. Perhaps, Kennedy takes the reverse point too far when he says that Arcadia should be associated with Gallus' poetry rather than Vergil's, but I think that this is more right than wrong - despite the fortuitous association of Arethusa with both Sicily and Arcadia. The negative point which Jenkyns stresses is important - if Arcadia was strongly associated with Vergil one would expect some awareness of that to be detectable in the Vergilian commentators and e.g. Calpurnius Eclogues. Also, important, is that we seem to be meant to feel the geography and atmosphere of Eclogue 10 as being quite different from that of the other Eclogues (Damon's song in Eclogue 9 also excepted).
Prior to publication of the Eclogues, it seems that Arcadian myths had already become important in Latin poetry.
One thinks in particular of the Milanion and Atalanta myth (set in Arcadia), which Gallus seems to have treated as programmatic before, at the latest, the composition of Eclogue 10.
We also seem to have lost some important Latin treatment of the Pan and Syrinx myth (again, presumably set in Arcadia): glanced at in Ecl. 2 (where Du Quesnay (1977) thought this reached Vergil from Philetas via Gallus). Can one take the case further that such a treatment might have been Gallus'? Ovid at Met. 1.689ff tells it as if it is already a hackneyed story - Argus falls asleep before Mercury is able to finish the story (Met. 1.700 'restabat verba referre...' and 1.713-4 'talia dicturus') and presumably that is why Mercury has chosen this story.
Note the resemblance of the opening words 'tum deus "Arcadiae..."' (Met. 1.689) to 'Pan deus Arcadiae' (Ecl. 10.26) (a clever adaptation - Ovid has separated 'Arcadiae' from 'deus' by making it the first word of the direct speech, but keeping everything in the same place in the metre - but is he adapting Vergil (if so, why?) or was Vergil adapting Gallus? Note then, the next words 'gelidis sub montibus' (Met. 1.689) with Tib. 2.4.8 'quam mallem in gelidis montibus lapis' - Gallus again? The preceding line in Tibullus, Tib. 2.4.7 'o ego ne possim tales sentire dolores' resembles Prop. 1.5.1 'Quid tibi vis, insane? meae sentire furores?' (addressed to Gallus, therefore, Gallus again?). Note also that Ovid's Syrinx is among the Hamadryads - 'inter Hamadryades' (a half pentameter?) (compare Ecl. 10.62 and Prop. 2.34.76 'laudatur facilis inter Hamadryades' (the 10th line of a ten line summary of the ten Eclogues); 'inter Hamadryadas' again at Ov. Met. 14.624 - see Kennedy (1982) for an argument that the Hamadryades are particularly associated with Gallus).
Note also, Servius ad Ecl. 10.26
'quem vidimus ipsi' utrum, quia praesentia numina agrestium, et ipse et fauni: an quia solent numina plerumque se rusticis offerre: unde est "satis est potuisse videri". notandum sane quod ea numina plerumque, quae amaverunt, dicit ad amatorem venire: nam Apollo amavit Daphnen, Pan Syringa, Silvanus Cupressum.
Servius ad Ecl. 10.28
'amor non talia curat' quasi expertus in Syringa loquitur.
Servius on Ecl. 10.26 is particularly interesting - why does Servius identify these particular amatory encounters among the very many of Apollo and the others of Pan (Echo, Luna / the moon and Pitys)? Is it a trace that the earlier commentators identified more particularly what Vergil alludes to - viz. the poetry of Gallus?
And Arethusa and Alpheus is an amatory story as much as (more than?) a bucolic one. It's of the god chasing nymph type (cf. e.g. Apollo / Daphne; Pan / Syrinx; Minos(not a god, but still...) / Britomartis). It's intriguing that Arethusa pops up again in Georgics 4 (as a huntress, with no bucolic association...?) and also that Aristaeus/Eurydice seems to be invented by being modelled on the god-chasing-nymph type scenario; and further that Aristaeus' Arcadian connections have been given an emphasis which was not necessarily demanded by the earlier tradition. Intriguing too that Calliope's song in Met. 5 - where Calliope is emphatically Gallus' Calliope, see Hinds (1987) discussion of Met. 5.344-5 as compared with Gallus Fr. 2.6-7 - reprises the Arethusa / Alpheus story (with Arethusa resembling the Arethusa of Geo. 4 ('quarum dea sustulit alto / fonte caput viridesque manu siccata capillos' Met. 5.574-5).
The connection between (i) Parthenius = the mountain in Arcadia so-named because of Atalanta and (ii) Parthenius = the Greek poet who taught both Gallus and Vergil is particularly intriguing - might that provide the key? I.e. could both Vergil's and Gallus' references to Arcadian myths be signals of their debt to Parthenius? Could, indeed, Parthenius have used the Atalanta myth programmatically before Gallus?
(3) Callimachean poetics in Eclogue 6
The below doesn't necessarily cut across your postulation of Vergil proposing a three-tier conception of poetic register, but it's a point which I don't think has ever been properly emphasised, namely how emphatically Vergil goes out of his way to show that Silenus' song is not in accordance with Callimachean precepts:
inflatum hesterno uenas, ut semper, Iaccho; 15
serta procul tantum capiti delapsa iacebant,
et grauis attrita pendebat cantharus ansa.
Ecl. 6.15-7
1. Compare:
tu satius memorem Musis imitere Philitan
et non inflati somnia Callimachi.
The metaphor ('inflatum' / 'inflati') that seems to be played with is indigestion - Callimachus' dream (i.e. the Aetia) was cultured and elegant because he wasn't stuffed full. Contrast Silenus. And then, refer back to Ecl. 6.4-5
'uellit, et admonuit: "Pastorem, Tityre, pinguis
pascere oportet ouis, deductum dicere carmen."'
Silenus has not followed Apollo's advice.
2. Callimachus either a teetotaller or, at worst, a drinker only in moderation (in metapoetic terms as well):
* Aet. Fr.178.11-12 Pfeiffer 'For he disliked greedy drinking of unmixed Thracian wine, and preferred a small cup' (of the guest from Icos, whose tastes Callimachus espouses)
* Antipater, AP 11.20 seems to be aimed at Callimachus
φεύγεθ᾽ ὅσοι λόκκας ἢ λοφνίδας ἢ καμασῆνας
ᾁδετε, ποιητῶν φῦλον ἀκανθολόγων,
οἳ τ᾽ ἐπέων κόσμον λελυγισμένον ἀσκήσαντες,
κρήνης ἐξ ἱερῆς πίνετε λιτὸν ὕδωρ.
σήμερον Ἀρχιλόχοιο καὶ ἄρσενος ἦμαρ Ὁμήρου
σπένδομεν ὁ κρητὴρ οὐ δέχεθ᾽ ὑδροπότας.
* as can be seen from the end of the Call. Hymn to Apollo 111-3
Δηοῖ δ᾽ οὐκ ἀπὸ παντὸς ὕδωρ φορέουσι Μέλισσαι,
῾̣̣̓λλ᾽ ἥτις καθαρή τε καὶ ἀχράαντος ἀνέρπει
πίδακος ἐξ ἱερῆς ὀλίγη λιβὰς ἄκρον ἄωτον.’
Silenus is the opposite - NB especially Ecl. 6.17.
3. Silenus' 'garland' has slipped far from his head: the metapoetical point is obvious. For metapoetic garlands see e.g. the 'serta' of Prop. 3.1.19 'mollia, Pegasides, date vestro serta poetae'.
4. 'attrita' seems to look to Apollo's warning to avoid the 'keleuthous / [atript]ous' Call. Aet. Fr. 1.27-8 (and cf. Antipater AP 7.409.5-6). Lucretius had already rendered '[atript]ous' with 'trita' 'loca nullius ante / trita solo' Lucr. 1.927-8. Silenus' tankard is 'attrita' - an exact aural equivalent but with the opposite meaning.