I'm sure you meant to say Manasseh rather than Benjamin, who was Joseph's brother rather than his son. I believe that Dan was left out because of the prophecies for the tribes in Gen 49, where their father Jacob tells them what will happen to each of them in the "last days."
My theory is that because Dan is said to be a judge of the people in the last days, he can't also be a witness, as those of the other tribes are. Because Dan is excluded, Manasseh is mentioned instead. He is in fact the firstborn of Joseph, and got his own special blessing at the same time.
With regard to spiritualizing scripture and taking prophecy figuratively, what is the criteria then for determining when we do or don't? In my opinion, the only safe way to approach it is to trust the plain text, unless context dictates otherwise.
A text without a context is a pretext, not a plain text! Biblical writings must be read -- Biblically. In terms of the whole message and pattern of the Bible. For example, a hypertext version of Apocalypse would be almost solid blue-underlined, since it is so densely linked with all that's gone before. You might almost call it an index to the rest of the Bible! For example, the "plot" is governed by the Ezekiel lectionary -- divide each book into 52 chunks, so as to read through it in a year, and the themes of Apocalypse consistently echo those in Ezekiel.
People who go in for plain-text proof-texts do things like "disproving" the Trinity, the God of the Bible. Or blending with fevered and overwrought imaginations to create deceptive and addictive "road-maps to the future."
I do find the different measures explaining the "error" of leaving out Dan in Rev 7 to be just plain weird. 12 tribes are 12 tribes. Nowhere in the Pentateuch do you find any kind of "replacement" stuff for Dan. Divvying up the land makes mention of a 1/2 tribe (Manasseh), but even then, it is clear what is going on. Revelation claims plainly and succinctly that these are "all the tribes of Israel" and then misses a tribe. I am forced to the position that either someone forgot something, or that we are given a hint here that apocalyptic language is MEANT to be interpreted symbolically.
Your point about how do we distinguish symbolic from literal is a good one, I believe. We do not, I believe, have hard and fast rules. I believe dispensationalism's greatest error is being unwilling to live without such a rigid rule and thus making up one and imposing it on the Scripture, even when the Scripture itself in the New Testament says (in effect) "No, that is NOT the rule for the interpretation of the Old Testament." It is true that one can spiritualize away almost anything if you run wild. However, that danger, true as it is, does not permit us to trump the Holy Spirit when He says clearly that the covenant promises extended in the context of a geopolitical ethnic nation state were really intended for the "true Israel" (OT terminology is "remnant"). Ephesian 3 claims furtherthat the NT has seen such a radical influx of non-ethnics into the remnant that the geopolitical ethnic exclusivity of the OT is, in fact, broken and done away with, so that "Israel" (or the "remnant" or the "church") is now a universal and many cultured family, and that ALL of the promises that many Jews (including Jesus own diciples)thought were to be fulfilled in a Jewish state are to be fulfilled through a stateless and universal people of God.
This is the beauty of covenant theology. It brings the gospel front and center as the message of the ENTIRE BIBLE and makes the person of Christ the central theme and message of the two testaments. I am filled with wonder as the promise expands in its scope and vision across the biblical timeline. This was the view, no matter what one's "millineal" views (and there were plenty of pre post and a mil views throughout history) of ALL the early church fathers, the confessing church through the medieval ages, and the protestant reformation. The radical disjunction of the testaments in dispensationalism is a late arrival on the scene for the simple reason that one will not come to it if the cardinal rule of the reformers in exigesis is used (scriptura scriptura interpres "scripture interprets scripture"). The bible will tell us how to interpret it if we will listen. We need not bring some man made rule of "literal whenever possible" to "help." Moreover, we make grevious errors when we insist on so doing.