There were 3 signs, not one. The symbolism proposed by the shape of the Hebrew letters fails because it's irrelevant for the other two signs.
I think there's more than just three signs. But I think they're all interrelated. For instance, though the Masoretic text doesn't clue the reader in to it, the text implies that when Moses places his hand in his bosom, at which time it becomes leprous, that hand still has the serpent-rod in it. If the serpent-rod represents the Torah scroll, then there will come a time in Israel's future when the Torah scroll will appear to be powerless to save Israel, leprous perhaps.
The symbolism proposed by the shape of the Hebrew letters fails because it's irrelevant for the other two signs.
Exodus 23:28 implies that the power God will send to drive out Israel's enemies is, get this, צרעה? The same word is found in Exodus 4 to describe what happens to the Torah scroll, Moses' serpent-rod, when placed in his bosom. For the non-Hebrew reader the word is "hornets" and "leprous." They appear to have not only a phonetic relationship, but they both, in some interrelated manner, appear to be a way God will defeat Israel's enemies.
Which segues into how the Hebrew word "
shakan" (to dwell) שכן relates to Moses placing his hand, with the serpent-rod in it, into his bosom, where the Torah scroll (or it's prototype manifestation) turns leprous: his dwelling place turns leprous.
When the Hebrew sages wonder out loud why "leprosy" and "hornets" use the same Hebrew consonants they sometimes say that a hornet sting is like the "burning" pain associated with the skin disease. Which is to say that, perhaps, God gets himself into something like a hornet's nest when he decides to enter time and tide to save Israel from her enemies: the burning thorn-bush, represented by the
shin, is a bush alight with flying seraphim, which, the seraphim, almost unbelievably, are associated with flames of fire. (Bronze is thought to look like flames when light hits it. And the word God uses when he tells Moses to add a fore-skene to his rod, is "
seraph" שרף. The sages wonder why Moses, and not God, chooses bronze? And why does Moses, not God, call the the "
seraph" a "
nachash"?)
The hornet's nest God gets himself into is the bush where his deepest Presence is protected by a hornet's nest of flying seraphim. You can't get to the Branch, the tree of life, unless you can get through the protecting angels who protect God just as they protect Israel even though something of their sting, their leprous nature, rubs off on everything they touch. (Perhaps this is why it's it's forbidden to touch the Torah scroll in this day and age and in its current state. A "
yad" must be used to point to the lambskin skene that's the Torah's fore-skene while it's in its leprous state.)
But this sign may have an additional meaning. From מקרב חיקך כלה (Tehillim 74:11) we learn that "hiding one's hand in one's bosom" is an expression denoting inactivity. Accordingly, this sign was intended to convey to the people the following message: "Placing one's hand into one's bosom" ---i.e., refraining from taking action ---can lead to one's destruction, if that is God's will. But if it is at God's behest that one assumes a stance of שב ואל תעשה and refrains from taking action, this can bring salvation.
The Hirsch Chumash, Shemos 4:7.
The referenced passage from
Tehillim 74:11 (Psalms 74:11), says (with verses 10 and 12 added):
9 We see not our signs:
There is no more any prophet:
Neither is there among us any that knoweth how long.
10 O God, how long shall the adversary reproach?
Shall the enemy blaspheme thy name for ever?
11 Why withdrawest thou thy hand, even thy right hand?
Pluck it out of thy bosom.
12 For God is my King of old,
Working salvation in the midst of the earth.
John