I will address this when I am able to get back to my computer, this is too important not to present the relevant Scripture with the response, so bear with me a bit. This tablet is bittersweet. Glad I can be on but not to be able to give a decent presentation has been killing me, lol.
The short answer/s would be...
Many think Paul died during one of his persecutions and it was then he was caught up to Heaven. This is post Pentecost so the Old Testament restriction from coming into God's presence in His eternal glory had been abolished by Christ.
Based on the same principle I do not view Enoch and Elijah to have gone to Heaven, but that Enoch died and was translated to Paradise ("...that he should not see death meaning he was spared the actual experience, not that he was raptured), and Elijah is simply caught up into the heavens, not Heaven itself.
Hebrews as well as Christ Himself seem to make it clear only the Son, in His incarnation, had seen God, and Christ being the first to be glorified would be an erroneous statement if in fact either of these men had gone to Heaven, as they would have had to have their sin atoned for and their bodies made suitable for the spiritual realm.
But we do see the spirits of men going into Sheol/Hades, as Christ affirms in Luke 16.
And I agree totally protos does not demand sequence but implies a "second," to which we look at the only two Resurrections taught in Scripture:
https://www.biblegateway.com/passage/?search=John+5:28-30&version=KJV
That is why the resurrection that follows the Millennial Kingdom is second. Just my opinion but I think it likely this will involve only the lost as they are designated as dead, a descriptor that cannot in sany way be applied to the regenerate (required before physical death) at that point as the current creation has passed out of existence.
When we look at Biblical use of protos we wouldnt impose a demand for a "second" when it describes priests, estates, seats, or even when used to describe the "first" commandment:
https://www.biblegateway.com/passag...om/passage/?search=John+5:28-30&version=KJVJV
...where the context makes it clear it stands alone.
We know the Covenant of Law is not the "first" Covenant, but as in Revelation 20 , in the context of the subject matter, for Israel (the ones exhorted to embrace the New Covenant) it is the highest in rank. Doesn't mean the Abrahamic Covenant is lesser, but we see protos distinguishing two Covenants only.
http://www.blueletterbible.org/Bible.cfm?b=Heb&c=8&t=KJV&p=0#s=t_conc_1141013
http://www.blueletterbible.org/Bible.cfm?b=Heb&c=8&t=KJV&p=0#s=t_conc_1141013
In Revelation 20 the meaning is further defined in that it is the resurrection of Life contrasted with the rest of the dead who are raised unto damnation one thousand years later.
And have to get on the road so I'll stop there.
God bless.