If I was to list the pieces of evidence which point to a Herodian origin for the gospel.
1. St Veronica is Berenice. I had photos from an archaeological dig in Banias and the cultic site Eusebius and later officials attest to for "Veronica" was on the front lawn of Bernice's castle (mentioned in Josephus). There are Herodian allusions in various chronographers regarding Veronica as a Herodian. This is the single most powerful piece of evidence but again I don't know how this translates into "Agrippa" being Mark other than her brother was so named.
2. Josephus's history of the Jewish War seems to unfold as Daniel Seventy Weeks fulfilment (even though Josephus himself seems to back date the prophesy fulfillment to Antiochus Epiphanes). Agrippa's speech and subsequent "cutting off" and "disappearing" more or less from the narrative seems to occupy a critical juncture in the narrative. The subsequent role of Agrippa as the "messiah" of Daniel 9:26 in Jewish and Christian exegesis of Daniel seems to dovetail but not be rooted in the Yosippon. But Origen knows of it to as well as other Christian sources (from memory). The Gospel of Mark is similarly rooted in Daniel chapter 9. I don't see how Mark the evangelist could have identified "Christ" with Jesus given the math previously mentioned. Either the crucifixion occurred in 21 CE or the standard model but not 60+ CE (where it would have to be in order to fulfil the prophesy). The death of James fits the time line but how could James have been the messiah but not Jesus? I am familiar enough with Marcionism that by the third century a garbled tradition that "Jesus" and "Christ" were separate individuals was associated with Marcionism. Irenaeus and the orthodox didn't like this understanding. "Jesus Christ" is posited as its replacement. But the Marcionites seem to have had an understanding that the Jewish prophesies applied to "Christ" who was a mighty potentate but Jesus's coming was unknown. Justin seems to have a similar understanding, shared to some degree by the Marcionites, that Jesus was "the man of war" of Exodus 15:2 who disguised himself as a humble man before revealing himself in glory by descending into the underworld and redeeming the souls who died without the Law (according to the Marcionite version of the myth). Justin's two advent theory assumes that "Christ" will reveal himself as a general or might man of war in "glory."
I have to get back to work but this is where my book got derailed. It wasn't that I assumed (as you claim) that Jesus had to be historical that broke down the theory. My difficulty was that I saw that the revelation of "Christ" in his "glory" as a man of war had to occur immediately following the crucifixion which I then (naively) assumed meant that Agrippa had to have eye-witnessed Jesus and the events of the gospel (misled also by Coptic traditions that Mark was an eyewitness). If we assume that the Jewish writings were understood to herald Christ but not Jesus at least some of the scriptures that Justin battled Marcion over assume the birth of the messiah. For instance this seminal passage that gets repurposed in various writings after Justin:
Begin we, therefore, to prove that the Birth of Christ was announced by prophets; as Isaiah (e.g., ) foretells, "Hear ye, house of David; no petty contest have ye with men, since God is proposing a struggle. Therefore God Himself will give you a sign; Behold, the virgin125 shall conceive, and bear a son, and ye shall call his name Emmanuel"126 (which is, interpreted, "God with us"127 ): "butter and honey shall he eat; "128 : "since, ere the child learn to call father or mother, he shall receive the power of Damascus and the spoils of Samaria, in opposition to the king of the Assyrians."129
[2] Accordingly the Jews say: Let us challenge that prediction of Isaiah, and let us institute a comparison whether, in the case of the Christ who is already come, there be applicable to Him, firstly, the name which Isaiah foretold, and (secondly) the signs of it130 which he announced of Him.
Well, then, Isaiah foretells that it behoves Him to be called Emmanuel; and that subsequently He is to take the power of Damascus and the spoils of Samaria, in opposition to the king of the Assyrians. "Now," say they, "that (Christ) of yours, who is come, neither was called by that name, nor engaged in warfare." But we, on the contrary, have thought they ought to be admonished to recall to mind the context of this passage as well. For subjoined is withal the interpretation of Emmanuel--"God with us"131 --in order that you may regard not the sound only of the name, but the sense too. For the Hebrew sound, which is Emmanuel, has an interpretation, which is, God with us. [3] Inquire, then, whether this speech, "God with us" (which is Emmanuel), be commonly applied to Christ ever since Christ's light has dawned, and I think you will not deny it. For they who out of Judaism believe in Christ, ever since their believing on Him, do, whenever they shall wish to say132 Emmanuel, signify that God is with us: and thus it is agreed that He who was ever predicted as Emmanuel is already come, because that which Emmanuel signifies is come--that is, "God with us." [4] Equally are they led by the sound of the name when they so understand "the power of Damascus," and "the spoils of Samaria," and "the kingdom of the Assyrians," as if they portended Christ as a warrior; not observing that Scripture premises, "since, ere the child learn to call father or mother, he shall receive the power of Damascus and the spoils of Samaria, in opposition to the king of the Assyrians." [5] For the first step is to look at the demonstration of His age, to see whether the age there indicated can possibly exhibit the Christ as already a man, not to say a general. Forsooth, by His babyish cry the infant would summon men to arms, and would give the signal of war not with clarion, but with rattle, and point out the foe, not from His charger's back or from a rampart, but from the back or neck of His suckler and nurse, and thus subdue Damascus and Samaria in place of the breast. [6] (It is another matter if, among you, infants rush out into battle,--oiled first, I suppose, to dry in the sun, and then armed with satchels and rationed on butter,--who are to know how to lance sooner than how to lacerate the bosom!)133 Certainly, if nature nowhere allows this,--(namely, ) to serve as a soldier before developing into manhood, to take "the power of Damascus" before knowing your father,--it follows that the pronouncement is visibly figurative.
In other words, I think the ur-Gospel had no mention of Jesus's birth. It developed later from scriptural material applied to "the Christ" by Jewish exegetes and for the most part argued by Justin to apply to Jesus. But could there have been an earlier tradition which assumed that Jesus came to the world as a supernatural being and became fully realized in a "man of war" a general a might potentate? The problem with this approach is that it forces the author to basically become a theologian bending evidence to suit the theory. If the theory is correct I would, in theory, only be following first and second century exegetes who did the same thing. But this is a big "IF." My opinion now is that it is better to avoid engaging in apologetics.
Unless some document or evidence for the cult of "Christ" being linked to Agrippa emerges, I think it best to avoid manipulating evidence in favor of a proposition that quite honestly has little supportive attestation. It doesn't mean that I don't think it might be possible, that Agrippa wrote the gospel and arranged a narrative to support his candidacy as "the Christ" secretly. I don't feel comfortable taking on the kind of role that is required to advocate this position - i.e. becoming the kind of person I detest, an "apologist" a virulent advocate for a position that doesn't really have a lot of supporting evidence and quite frankly might be entirely incorrect. I've been in love more times than I care to remember. And love kept me cool in July and warm in September. It may not have lasted, but each time I thought it was heaven. You name it, I've been there and back. Looking for someone who I'd be faithful to.
When you're young you think every time you feel inspired it must be "true love" it must be the one. Sometimes love is just irrational arousal.