Some Ancient Answers
In interpreting the oldest surviving mythography featuring Pandora, Pausanias' Description of Greece, penned almost a millennium after Hesiod's texts, says plainly that "Pandora was the first woman" but he also expands upon this with the statement that "before Pandora was produced there was as yet no womankind." Writing about a century before Pausanias, the Roman author Hyginus says in his Fabulae that Pandora's daughter Pyrrha was the first mortal to be born.
Taking those assertions simply at face value, it would appear that these writers understand the answer to your query as your "Possibility 2": that there indeed were female entities similar to human women in existence before Pandora—as Hesiod tells us in his Works and Days, she was after all modelled on the immortal goddesses, to have a face like theirs—but, as you summarise it, she was the prototype for all others to come thereafter, the next one being her own daughter who would grow up to survive the Flood.
A Closer Look at Those Answers
Such absoluteness is difficult to square with the genealogies of characters in the mythology with which we are supplied by these very same writers. For, thanks to them, and others both before and after, we know of numerous female characters who would have predated Pyrrha, or been contemporaneous with her, who seem to live very human lives on earth, among the men of the world. They are in fact the ancestresses of the majority of the personages in Greco-Roman myth. A good number of them do descend from Pandora, but by no means are her offspring the majority.
Still, even though they seem to have been indistinguishable from human women, it actually is plausible to classify the female characters as "other creatures" as you do in your "Possibility 1". The vast majority of these contemporaries of Pandora we know to have been different sorts of nymphs (usually Oceanids and Naiads), or minor goddesses of some kind. A few here and there are the daughters of giants or Autokhthones [Autochthons, i.e. people sprung directly from the ground]. Some of the rest would be the product of a second or third generation of the aforementioned, with yet more nymphs or such entities as their mothers.
So indeed, according to the genealogies provided to us, just as you say in your "Possibility 3", the generations of "men" prior to the Age of Heroes did include females therein, but one could, somewhat reasonably, argue that they 'were so different from our concept of human female that they could not be called "women".' Contrary to what Hyginus says, there were indeed mortals born among these earlier generations of female human-like beings.
Men of Bronze, Silver and Gold
With all those nitpicks and qualifiers aside, the mythology actually contains remarkable internal consistency, starting off, in fact, with Hesiod, who provides solutions to some of these quandaries. In his myth of the Five Ages (or, more properly, Five Breeds/Races) of Man, he says that the first two phases or breeds of men, the Golden and Silver Races, were crafted by "they who dwell on Olympus".
The third breed, the race of bronze men, was "sprung from ash-trees". Apollodorus does mention the thing that you note about the destruction of this race: it was brought on by the Flood, which covered the Earth in the time of Pandora's daughter Pyrrha.
In speaking of Talos, the brazen giant employed by Minos as guardian over the island of Crete, both Apollodorus (Bibliotheca 1.9.26) and Apollonius Rhodius (Argonautica 4.1638-1644) mention the idea that this giant was a survivor of the Flood from the Bronze Age. This suggests that the races of the first three ages were huge metallic androids fabricated by the gods out of tougher elements than the soft tissues that we—modern humankind—would later be constituted of.
The First of Her Kind
This would explain why the creation of Pandora might be so impactful in a world already populated by thousands of Oceanids and numerous other sorts of nymphs who were already consorting with these metal men, Autokhthones, and early descendants of the gods and Titans. Pandora would genuinely have been the first of her kind: a purely human woman made of flesh, blood and bone, from scratch, unlike the previous renditions of female entities.
Neither does the story of Pandora's creation contradict that of Prometheus' creation of mankind. The latter is supposed to be precisely that: a race of only male humans who, going by the story, supposedly lived in blissful ease and chillax mode until the first female version of them came along and messed everything up, which is exactly what Zeus intended when he commissioned Pandora's construction.
Timeline
The general flow of the chronology goes as follows, pretty much as you've laid it out in your Question:
- Prometheus makes men. (Or he and his brother Epimetheus are tasked
with distributing different features among all earth-dwelling
creatures. Epimetheus bungles the job by giving all the cool and
protective qualities and abilities to the animals, leaving nothing
for mankind.)
- Prometheus tricks Zeus into accepting, together with the rest of the
gods, the inferior part of the sacrifice that humans will from now
on offer to them. Zeus indignantly takes fire away from men.
- Prometheus steals the fire from heaven and gives it back to men. (He
does this partly to compensate for their lack of gifts due to
Epimetheus' previous botching of their work.)
- Zeus has Prometheus nailed to Mt Caucasus in Scythia. In the meantime he orders
the creation of the first human woman, Pandora, whom Epimetheus
marries, against Prometheus' advice. From the inside of a jar,
Pandora releases the multitudes of evils which plague the
human race to this day.
- In the time of Prometheus' son Deukalion [Deucalion] and of
Epimetheus' and Pandora's daughter Pyrrha, Zeus is fed up with the
bread-eating race created by Prometheus and sends a worldwide flood to
wipe them out.
- Deukalion and Pyrrha, having been assisted by Prometheus' advice,
survive the Flood and re-people the world both with their own
descendants and with new humans created by them throwing a bunch of
stones.
A Potential Glitch
The only clash of details to which I see some difficulty in finding a ready answer is how Prometheus' human creation fits in to the earlier part of the myth of the Five Ages of Man. Almost every account of the manner in which Prometheus modelled the first men has him fashioning them out of earth and water, just like Hesiod says that the smith-god Hephaistos [Hephaestus] built Pandora per Zeus's instructions.
One might hazard the guess that these men were the beginning of the Age of Heroes, but going by the chronology, these guys would have been annihilated in the Flood and replaced by new people sprung from stones or descended from Deukalion and Pyrrha.
Death and Mortality
As far as I know there is no ancient source that explicitly mentions Pandora's death or necessarily refers to her as a mortal, although to me this certainly seems to be the general implication, the closest statement to that effect being Hyginus appearing to indicate that Pandora's daughter Pyrrha inherited mortality from her mother. Modern commentary on this likewise tends to interpret Pandora as having been mortal, such as in the first sentence of Theoi.com's article on her.
Hesiod does not seem to have understood Death to be one of the malevolent things released from Pandora's jar, especially if we take his myth of the Five Ages as compatible with this story, since he talks about men dying prior to Pandora's time-period. The contents of the jar seem to have had more to do with life's difficulties rather than mortality. In Works and Days, they are listed as kakoi, "ills" or "troubles" (bad stuff); hard toil; and heavy sickness (plagues and the like). Going by that understanding, prior to Pandora's creation, men did die but their lives were not characterised by pain, disease, and the general struggle to survive.
And So Finally...
To go to the central issue, this would be the best interpretation I can draw from the material we have, beyond simply reiterating (as in the first section above) the words of Pausanias and Hyginus:
Pandora was the first genuinely human woman, being zero % nymph, goddess, Titaness, giantess, or Autokhton; nor a descendant of any of the earlier golden, silver or bronze men.
