It seems pretty clear to me:

It always seems to me from reading scripture that it is a misconception to believe that as soon as we die, if we are saved, then we go to heaven. It would seem that instead we wait in Sheol in Abraham's Bosom until Yahushua has prepared a place. This would seem to confirm that. He is the only one that has been there as of yet. But, when he returns, then he will take us, the saved dead being the first to rise, and the living the next.

I suspect that this verse is context sensitive in that Yehushua was instructing Nicodemus on being 'born from above'. Also we are still in a pre-crucifixion scenario. Nicodemus does not believe 'earthly things' said to him, so how can Yehushua explain the heavenly things. In the context here, the references to heaven are about the spiritual aspects of pistueo and its non-physical origins.

Nicodemus is, of course, a very confused man, he is a high official in the sanhedrin, but he would appear to be a physicalist and is thus confused that Yehushua, whom he probably knew was excellently qualified to be messiah, was not supporting the religious status quo.

It was normal for the sanhedrin to pronounce all claims to 'messiahship' as false, based upon OC prophesy and temple records which together were very difficult for a claimant to get around, in fact they were impossible because the line of 'kings' was cursed. So even if Joseph was direct heir to the throne of David, his son would not be able to claim kingship because the bloodline was cursed. That is, of course, until a virgin gave birth....

Heaven , in the contex of John 3 v 13 is the source of all things spiritual, and, that spiritual authority rests uniquely with Yehushua until the Spirit is released post resurrection (John 20 v 22).

I really like what I have read and I don't want to simplify or take away from what has been said but could there be a simpler explanation? Could it be that people just assume that Enoch and Elijah were taken to the kingdom of heaven?

(The Scriptures 1998+) And it came to be, as they continued on and spoke, that see, a chariot of fire with horses of fire which separated the two of them. And Ěliyahu went up by a whirlwind into the heavens.

that word "heavens" is shamayim

H8064
שׁמה שׁמים
shâmayim shâmeh
shaw-mah'-yim, shaw-meh'
The second form being dual of an unused singular; from an unused root meaning to be lofty; the sky (as aloft; the dual perhaps alluding to the visible arch in which the clouds move, as well as to the higher ether where the celestial bodies revolve): - air, X astrologer, heaven (-s).

could it be that a better translation could at this point be sky?

Then let's move to Enoch/Hanok
Gen 5:24 And Ḥanoḵ walked with Elohim. Then he was no more, for Elohim took him.

That's not very specific.

In light of the fact that these witnesses are killed...It would seem that they have not recieved resurrected bodies. Is it possible for an unresurrected to enter the kingdom of heaven?

Here is something even simpler. If indeed the two witnesses did enter the kingdom of heaven it was not of their own power or volition. In both cases Yahuweh took them wherever they went. Yahushua through his own power and volition has descended and ascended many times throughout the scriptures. Look to his busy ways in the covenant scriptures.

As I said before I am not trying to oversimplify the issue and I delighted in your brilliant answers, but on the simpler side of the issue I think these explanations may cover the problem too for those who are not as adept or as enlightened.

Well, it seems that the dead know not anything and remain in the grave, Hanok and Eliyah were raptured, and only Messiyah Yahushua went back to Heaven after spending time here. So the obvious thing is that Hanok and Eliyah have been taken somewhere/sometime else other than heaven, and that any thinking that good people go to heaven when they die is false. Otherwise what is the purpose of the ressurection?