Question:
Doesn't Num 23:19 "G-d is not a man that he should lie" prove to us that the messianic argument regarding an incarnation of (at least part) of the Creator as null and void.
 
Answer:
First and foremost I am not a Messianic but an Orthodox Jew and I wholeheartedly agree that the idea of the almighty being ONLY a man is un-biblical in every way which is what Num 23:19 is stating (G-d is not a man that he should lie), not that the verse is stating that the G-d of Israel cannot temporarily become or embody a man if he chooses to. (An idea I DO NOT embrace).
(note: a belief that would include any other deity would be 100% idolatrous.)
 
I also personally do not think that the creator would do either mainly because I subscribe to the philosophic arguments mentioned by the Rambam in Moreh Nevuchim. (In not limiting the Creator) But this was only the Rambam's assessment, not halacha.
 
That being said, there have been many Rabbis who did believe that the almighty did appear to his creation in the shape of men and angels, like Rav Saadia Gaon in his theory of “Created Glory."
 
And this is clearly seen within the literal narrative of Tanach itself.
(Joshua 5:13, Genesis 31:11–13, Exodus 3:2–6, Genesis 19:1-5, Ezekiel 1:26-28, 2:1-5, Genesis 32:24-30 etc.)
Again, I believe that any instance of biblical anthropomorphism is NEVER to be taken literally, IE The idea of Hashem having a hand (Deut 26:8) or a foot (Is 66:1) or even appearing as an angel or man, and explain the instances when the almighty does textually appears to man in any other form other than spirit as an illustrated dream being had by the individual. (Ie not to be taken literal) as the Rambam does by explaining that these instances appeared only while the person was dreaming. (Guide II, 41)
 
But friends this is a philosophical argument, not a theological one (ie not biblical).
 
I think Rabbi Bahya puts its the clearest.
 
Rabbi Bahya ibn Pakuda writes
"The foolish and simple person will conceive of the creator in accordance with the literal sense of the Scriptural phrase. And if he assumes the obligation of serving his G-d and strives to labour for his sake, he is excused for his ignorance and lack of understanding. A man is judged only on the basis of his intellectual ability, his powers of discrimination, and his readiness to act."
Hovot haLevavot 1:10 (Duties of the Heart).
 
Heretic and Idolater are halachic statuses developed by the Sanhedrin (beit din hagadol) to be able to adjudicate the theological enemies of Klal Yisrael accordingly, the reason that I say they (Messianic Jews who keep Torah and Halacha) are neither is because the Sanhedrin DID NOT ever establish a clause that would include (the original) messianic Jews (that 1%) as either kofrim, apikorsim, minim or ovdei kochavim.
(This is assuming they accept Written and Oral Law and just happen to believe that the G-d of Israel embodied a man for 33 years, even if they split the One G-d into different manifestations of that same one G-d (like the trinity).
 
 That being said, I hope you didn't miss the point I was heading to.
That by this acknowledgment of messianic beliefs THIS will only encourage Messianics to enter traditional orthodoxy and ultimately abandon their messianism as a whole.
 
How?
I've just demonstrated that by understanding Tanach literally, biblically and (regarding Chazal) halichicly, Messainics would not be performing a grave offense and could not be labeled as idolatrous or heretical.
Which if they then accept this argument would also have to acknowledge and accept that by a Jew choosing NOT to embrace their beliefs that we just explained are not literally condoned or condemned in biblical nor halachic Judaism they MUST also have to admit that the same Torah could not condemn a Jew who chooses NOT to accept or believe in some supposed heavenly incarnation or even the notion of a messiah that also does not literally appear in Torah. (Ie their is no command in Torah to even believe in some Messiah).
 
 
 
In other words, the way you destroy Nazerene Messianism is by proving that according to Torah you do NOT have to believe in what they teach in order to be saved/understanding scripture correctly/viewed as obedient by your G-d.
So if you don't NEED Jesus (according to the biblical narrative) Messianic Judaism ceases to exist.