I have been puzzled by the trees in the corners of the images of the throne almost since I first saw it. I noticed that with the human figure they are partially 'underground' (like asparagus spears I guess) while with the lion they are fully uncovered. This can't be accidental. I have struggled with every possible explanation - maybe the 'ground' was added later, maybe it was an 'accident' or means nothing. In the end when we uncovered the connection with Zech 6:12 and your analysis of the shades of meaning of tsemah I think I have come up with a plausible explanation, one which further solidifies the presence of the passage from Zechariah.
As I have mentioned many times the four hiyyoth can be grouped in terms of those with tamar trees (the human and the lion) and those who do not (the bull and the eagle). If the palms were purely ornamental one would have expected them to appear at each corner of the right and left sides of the throne like legs on a chair. However they strangely only appear on the left and the lower back images. Indeed even here they are not exactly the same. The human figure is accompanied by palm trees with only their top halves peaking out of the ground. Only when we get to the lion do we see the palm trees fully erect.
It is a lot easier to explain why there are no palm trees accompanying the eagle. It stands above the lion so if the 'height' here is meant to emphasize the eagle soaring through the clouds it is no wonder why no palms should be present. Yet the bull remains an enigma. Where is the bull? One could argue that it is a desert but bull's naturally don't appear there or in the sea (the only suggestion which might make sense with regards to an absence of trees). So we have to leave the bull for a moment and move on to the two other images which HAVE palm trees present.
When you look at the human and the lion figures one after another (as they appear on the throne) it is obvious that the tamar is meant to be depicted as growing. First it is in the ground then it rises to the surface in its full form. The gematria of TMR = ShMSh has to be at play here. The tree is the sun which rises from the depths to finally appear as the Rising Sun in the figure of the eagle with its wings almost touching the edge of the sun disk.
This means in effect that the bull has to be representing its presence in the underworld, a completely appropriate place for such an animal in Egypt as Apis is always depicted as residing there.
When you start to see it as a progression of the Rising Sun (exactly as described in Zech 6:12) coupled with the fact that Marcus Agrippa must have been the historical 'St. Mark' who sat here in 'the (Samaritan) year' of 38 CE it all starts to come together. There is a story unfolding on ALL SIDES of the throne. We begin with the image of the Akedah on the back rest - this is an acknowledgment that little Mark was spared by Jesus' sacrifice. He is the new Isaac, Jesus is the Creator who in offering himself up is at once repenting from his original creation and setting forth the mechanisms for the perfect work (Deut 32:4) to be finally established.
This is Marcus Agrippa who is claiming at once to be the new aeon. There is a clear sense that Agrippa claims to have been 'resurrected' from his prison cell in Jerusalem (Philo Embassy to Gaius). This mean that we leave the back of the throne and head to our left where the bull appears. The bull is that experience. In Egyptian lore Apis is connected with the resurrection (it might have something also to do with the golden calf). Whatever the case we are meant to go from right to left here (from the perspective of the figure sitting in the throne) and onto the human figure. If the bull was 37 CE the human represents the present year of 38 CE when the enthronement in Alexandria was taking place. The human corresponds to Mark himself (this was apparently known to the person who made the top piece later). He has nine stars around him to emphasize the connection with Zech 6:12 hidden in the writing on the back rest. The shofar is/are being sounded by angels to signal the Jubilee. Yet the vision makes specific mention of 'the sprout' so it is important to notice the palm trees here just popping out of the ground.
When we move on to the lion the trees are fully erect and likely represent Agrippa's future voyage to Palestine where he receives his grandfather's kingdom. The lion is always a symbol of royal power. It is also always connected with St. Mark. The tree which stands under him with thirteen fruit likely signifies the letter M (the thirteenth letter) which in turn has a value of 640 also - m (40) + [closed] M (600) and of course is the first initial of Marcus' name.
The eagle which appears with the book with the equidistant cross also represents Mark but in a future age when he writes out the new Torah (the gospel). It represents the new Rising Sun as you point out from Malachi and let's not forget Deuteronomy, 32.11-12. "As an eagle awakens its nest, hovering over its fledglings, it spreads its wings, taking them and carrying them on its pinions. [So] the Lord guided them [the Israelites] alone."
Irenaeus while following a different order nevertheless also maintains a chronological order:
For the cherubim, too, were four-faced, and their faces were images of the dispensation of the Son of God. For, [as the Scripture] says, "The first living creature was like a lion," symbolizing His effectual working, His leadership, and royal power; the second was like a calf, signifying [His] sacrificial and sacerdotal order; but "the third had, as it were, the face as of a man,"--an evident description of His advent as a human being; "the fourth was like a flying eagle," pointing out the gift of the Spirit hovering with His wings over the Church. (AH 2:11:8)
All that he does in the next line is to emphasize that it is Jesus not Mark who sits in between these images of the four (pseudo) evangelists
And therefore the Gospels are in accord with these things, among which Christ Jesus is seated. (ibid)
Irenaeus maintains this same chronological order - lion, calf, human, eagle - in the three references which close the section:
For that according to John ... is that Gospel full of all confidence [i.e. like a lion], for such is His person. But that according to Luke, taking up [His] priestly character, commenced with Zacharias the priest offering sacrifice to God. For now was made ready the fatted calf, about to be immolated for the finding again of the younger son. Matthew, again ... is the Gospel of His humanity;for which reason it is, too, that [the character of] a humble and meek man is kept up through the whole Gospel. Mark, on the other hand, commences with [a reference to] the prophetical spirit coming down from on high to men ... pointing to the winged aspect of the Gospel; and on this account he made a compendious and cursory narrative, for such is the prophetical character. (ibid)
and again
And the Word of God Himself used to converse with the ante-Mosaic patriarchs, in accordance with His divinity and glory [i.e. the lion]; but for those under the law he instituted a sacerdotal and liturgical service [i.e. the bull]. Afterwards, being made man for us, He sent the gift of the celestial Spirit over all the earth, protecting us with His wings.
and again
Such, then, as was the course followed by the Son of God, so was also the form of the living creatures; and such as was the form of the living creatures, so was also the character of the Gospel. For the living creatures are quadriform, and the Gospel is quadriform, as is also the course followed by the Lord. For this reason were four principal (kaqolikai) covenants given to the human race:(3) one, prior to the deluge, under Adam [the lion?]; the second, that after the deluge, under Noah [the calf?]; the third, the giving of the law, under Moses [the man (of God?)]; the fourth, that which renovates man, and sums up all things in itself by means of the Gospel, raising and bearing men upon its wings into the heavenly kingdom.
I CAN'T BUT HELP THINK THAT IRENAEUS MUST HAVE BEEN LOOKING AT THE THRONE GIVEN THE REPEATED REFERENCE TO THE EAGLE SOARING ABOUT EVERYTHING ELSE. YOU SIMPLY CAN'T GET THAT FROM THE SCRIPTURAL REFERENCES ALONE.
All Irenaeus has done is started at the back with the lion and went counter clockwise (as I am suggesting). Starting with the lion you get lion, calf, man, eagle. If however you start at the right hand side you get calf, man, lion and eagle as I have suggested.
I think both systems go back to the same throne. Mine just happens to be the true one. Irenaeus is offering up dysinformation.