Good to hear from you again. To take your last question first, I do remember hearing about Mr. Missler from his days at Calvary Chapel in Costa Mesa CA (I did my graduate work at Talbot Theological and then at UC Irvine in that general geographic area); on his teachings, I am not a particular fan. Long story short, my approach is to start with the Bible, study the Bible, and end with the Bible. The importation of external "knowledge", no matter how deep and no matter how interesting, seldom seems to fail to engender error on at least some level (pace the physics dept., we do not really even know for certain if there is such a thing as inter-dimensional space -- how could we know that this is where angels come from!?).
As to your question on the Nephilim, there are things we know about angels; there are things we can reasonably deduce; but there are some things we cannot really know this side of eternity. Genesis chapter six (along with the supporting New Testament passages) is very specific about the fact of (fallen) angelic cohabitation with human women; it says nothing about the mechanics. Given that angels are comparable to human beings in their general physical shape and appearance (and that there are apparently female angels; see the link), positing that the mechanics and equipment are also comparable is no great stretch of logic. The main difference between us, of course, is that angels do not possess physical bodies. As I have often had occasion to explain, the fact that angels are "spirit beings" does not mean that they do not have substance. Indeed they do. After all, angels are recognizable as creatures, inhabit a single time and a very definite space, and must "physically" transit from point A to point B (even if the means and speed by which they do so is miraculous in human terms). Moreover, the Bible is filled with examples of angels affecting the material world. The angels who kept vigil at Christ's tomb rolled away the stone which sealed it. Thus, angels can affect the material world for good; they can also affect it for evil (as in the destruction of Job's family and livestock). They are spirits, but they are confined to a particular shape; they have what I term "spiritual corporeality" as opposed to being spirits intimately connected to a physical body in the case of human beings (see the link: "The Creation of Man").
We live in era when human science is beginning to manipulate the genome in all sorts of frightening ways. Since no one would be terribly surprised to hear of it if one day "scientists" somehow managed to synthetically construct a viable substitute for male seed in lower animals, it seems to me that the idea of angels (who are, after all, far more powerful, capable, and knowledgeable than we are – and who have been around since before the six day re-construction of the earth in Genesis chapter one) being able to do roughly the same thing should not occasion disbelief. That is as far as I should wish to take things except to note the following scripture:
Although you know all these things, I want to remind you that though the Lord saved [all] His people from the land of Egypt at the first, in the end He destroyed those who proved unfaithful, and the angels who did not keep to their own realm but deserted their proper habitation He has imprisoned with everlasting chains in the gloom below (i.e., in the Abyss) in anticipation of the judgment of that great day, just as He did with Sodom and Gomorrah and their neighboring cities, for they all prostituted themselves in the same manner as these [angels] did, having pursued sexual relations [lit. "flesh"] that were inappropriate for them (i.e., outside of the natural order). [And so it is that] they have been appointed an example, and incurred the penalty of eternal fire.
Jude here does indicate that the nature of the union of angels to human woman is analogous in its inappropriateness (and its violation of God's natural order of things) to that of human men to human men. It seems likely, therefore, that in both instances we have two groups on each side of the equation who are capable of normal sexual relations but who have violated God's order in having illegitimate relations with an inappropriate group (outside of the proper kind in the first instance; outside of the proper sex in the second).
Angels, it does seem, do not procreate their own kind. While scripture does not say so, the conclusion to which I and many others have come on this matter is that God created the entirety of angelic kind at once, and no more have ever or will ever be created (after all, angels do not seem to be "choosing" for or against God at this point; they very clearly have already "chosen" and that must mean that no more are being produced who must subsequently choose sides).
What is lost even on many Christians is that human kind is likewise a consummate whole, and that the number of human beings, body and spirit, ordained in the Plan of God, a perfect number that "had to be" if the world were to be created, is likewise a precise, fixed total (see the links: "Faith: What is it?", and "Free-will faith and the Will of God"). The difference, of course, is that angels were created all at once; human beings come into the world through the creative act of God individually and over the seven thousand years of human history (mediated, of course, through the act of physical, human procreation -- but no life comes into being without God's provision of a human spirit at birth; see the link: "Life Begins at Birth").
To sum up, we know that angels can affect matter, we know they can accomplish all sorts of things which to human beings seem miraculous, and we know from Genesis six that fallen angels did cohabitate with human women (cf. Jude 1:5-7; 2Pet.2:4-10). You are correct in your assumption that I see antichrist as the result of precisely the same sort of operation: he is described in Genesis chapter three as "your seed" (i.e., the seed of the serpent possessed by the devil). Exactly how fallen angels are able to accomplish this travesty of nature (for which those involved in the Genesis six attack have been confined in darkness in the Abyss) the Bible does not say – for what would seem to be compelling reasons: beyond the obvious apparent mechanics of such unions, the "scientific" explanation would no doubt be beyond present science's ability to comprehend.
I hope this is of some help to you. Kudos to you for actually reading the links provided in emails past! I put them in my responses for a reason, but, like the verse citations, I have a feeling that following these things up is for most more of the exception than the rule.
In our dear Lord Jesus Christ for whose return we breathlessly wait,