I believe all of those options are possible.
[This paragraph is partly wrong; I should correct it sometime. The rest should be correct. The vocative on -e only came to Latin relatively late, although it had become the standard by the classical age. In older Latin, the nominative -us functioned as the vocative—i.e. Latin had no separate vocative at first, for all other forms of the vocative were and always remained identical to those of the nominative, like femina, pater, etc. Perhaps it was borrowed from the Greek vocative on -e of the same declension as in Latin?]
And -e was still not used, or not often used, with certain specific words, like deus, whose vocative remained deus throughout the classical age and beyond, I believe, probably because dee looked and sounded odd. Ritter, through Eden (in his commentary on Aeneid VIII.77), says that the vocative popule did not occur before Quintilian, but that is not quite true, unless there are textual errors in Ovid and Cicero, which seems very unlikely. But popule does seem to be rare.
I think it makes the most sense to say that it was possible for classical authors to use the nominative where one would expect a vocative, both in Greek and in Latin, although it was uncommon to do so. I suspect it was mostly used in poetry or high oratory, and in ancient texts. Perhaps some instances could be explained away as appositions to an unmentioned nominative subject, but I believe others cannot (see below).
At any rate, in those examples that could be explained away, the construction is such that the author might have used either a vocative or a nominative; for I believe basically any noun (substantive or adjective) that refers to the same thing as the subject of a verb in the second person can be written in the vocative in Latin. Does it really make any difference whether we translate Horace's laudas insanus/insane trilibrem mullum as "you, being insane, praise a three-pound mullet", or "you, o insane one, praise a three-pound mullet"? Isn't the meaning always the same?
We may get the impression that the Romans consistently used the nominative when it was an apposition and the vocative when it was an address, but that is only because we add commas around a vocative but not around a nominative, and either case is normally possible when the verb is in the second person. Can you guess which case Horace really used in the example above?
Priscianus describes the phaenomenon thus:
omnia autem pronomina, quae uocari possunt, similem habent nominatiuo uocatiuum excepto mi pro mee. nec mirum, cum etiam (0208) nomina pleraque apud Latinos, ut diximus, eosdem habent nominatiuos et uocatiuos. Atticis quoque mos est nominatiuos pro uocatiuis proferre.
Homerus: « ἠέλιός θ᾽ ὃς πάντ᾽ ἐφορᾷς καὶ πάντ᾽ ἐπακούεις.
quem secutus Virgilius in VIII:
corniger Hesperidum fluuius regnator aquarum
fluuius pro fluuie dixit, et Lucanus in II:
degener o populus, uix saecula longa decorum
sic meruisse uiris,
So Priscianus says the nominative could be used instead of the vocative.
The example he gives from Lucanus has "o" and couldn't be taken as appositive.
The full quotation from Virgil that Priscianus mentions is as follows, from Aeneid VIII.77:
quo te cumque lacus miserantem incommoda nostra
fonte tenet, quocumque solo pulcherrimus exis, 75
semper honore meo, semper celebrabere donis
corniger Hesperidum fluvius regnator aquarum.
The Loeb translation is as follows:
"In whatever spring your water contains you as you pity our travails, from whatever soil you flow forth in all your beauty, ever with my offerings, ever with my gifts, you will be graced, horned stream, lord of Hesperian waters."
I suppose you could see this as appositive, but I'd rather see it as a free choice between nominative and vocative, especially in poetry.
Servius, too, mentions the possibility of a nominativus pro vocativo, in his commentary on Aeneid XI.464. The passage from the Aeneid:
“tu, Voluse, armari Volscorum edice maniplis,
duc” ait “et Rutulos. equitem Messapus in armis,
et cum fratre Coras latis diffundite campis. 465
" 'Messapus' autem ut diceret, vitavit ὁμοιοτέλευτον: nam vitiosum erat 'Voluse' 'edice' 'Messape'. ergo 'Messapus' aut antiquus vocativus est, ut 'Hesperidum fluvius regnator aquarum', item 'socer arma Latinus habeto': aut certe nominativus est pro vocativo. quamquam possit etiam nominativus esse, ut sit 'Messapus et Coras equitem diffundite' pro 'diffundant': melior tamen est sensus superior. "
He first offers the possibility that archaic vocatives on -us existed, like fluvius and Latinus in his examples. His second suggestion is that it may be a nominativus pro vocativo. His third suggestion is that there is an error in the text, but he says the "interpretation above" ("sensus superior") is the better one; this suggests that a nominativus pro vocativo and an archaic vocative more or less amount to the same thing (the "interpretation above") phrased in two different ways, which would make sense and would agree with Priscian.
In Servius's second example, Aeneas is speaking in the presence of Latinus, but he is speaking in the third person (habeto): "Latinus, my father-in-law, is to keep the sword" (Loeb translation). This example does not seem relevant.
A relevant example from Livius:
audi, inquit, Iuppiter, audi pater patrate populi Albani, audi tu, populus Albanus (Livius, Ab Urbe Condita I.24.7)
"Listen, he said, Jupiter, listen, pater patratus of the Alban people, listen you, the Alban people."
Even though some interpretors would want to explain this as an appositive, it seems to be a classic example of a situation in which one would use a vocative. Populus Albanus is parallel with Iuppiter and pater patrate (a certain official), which has the vocative ending -e. It may be so that the priest is addressing the Alban People in the incarnation of some dignitary or leader, but then that official still is the Alban people, so a vocative would by all means be appropriate.
And Horatius, Odes I.2.43:
sive mutata iuvenem figura
ales in terris imitaris almae
filius Maiae patiens vocari
"Or you, o winged son of kindly Maia, if you take on the shape of a young man on earth and are willing to be called Caesar’s avenger" (Loeb translation). The speaker is exhorting Mercurius. You could explain it away, but why would you?