Classical: On the subject of a sentence with 連用形/て clauses + ば condicional

Question

I'm having some trouble with this beautiful passage from the Kana Preface to the Kokinshū, which is talking about us (spaced for clarity):

 1 人まろ なくなりにたれど、
 2 うたの こと ゝどまれるかな。
 3. たとひ とき うつり こと さり、
 4. たのしび かなしび ゆきかふとも、
 5. この うたの もじ あるをや。
 6. あをやぎの いと たえず、
 7. まつの はの ちりうせず して、
 8. まさきのかづら ながく つたはり、
 9. とりのあと ひさしく とどまれらば、
10. うたの さまを（も） しり、
11. ことの こゝろを えたらむ人は、
12. おほぞらの 月を みるがごとくに、
13. いにしへを あふぎて いまを こひざらめかも。

The Iwanami and Shogakukan anotated editions tell us two pieces of cultural context needed to understand it:

1. The modifier clauses starting each line in 6-9 are traditional images (序詞) for the things that follow: green willow threads for "not ceasing", pine needles for "not scattering", the masaki-no-kazura vine for "reaching long", and bird tracks for "lasting a long time". It may seem strange that bird tracks would be an image for 久しく とどまる, but this is due to the fact that:
2. 鳥の跡 is a conventional metaphor for "written characters" (due to the legend that Cangjie invented characters by imitating the footprints of plovers). So this particular image also echoes the point of line 5 above.

(grammar note: if とどまれらば seems weird, it's because ら here is the 未然形 of the perfective/progressive suffix, り.)

Now here's Lamarre's translation:

Although Hitomaro has gone, the acts of songs have remained. Even though eras shift and deeds pass, and delights and sorrows come and go, the characters (moji) of songs continue. Should they be retained as changeless as birdtracks and transmitted as long as rampant vines, just as the evergreen needles never scatter and vanish, just as the threads of green willows always trail, then the people who know the designs of songs and obtain the hearts of words surely will look up to the high ages and yearn for this day, just as we look to the moon in the great heavens. [emphasis mine]

A bit liberal in some aspects, and it glosses over the "bird tracks=characters" motif, but my question is regarding the parts in bold. I don't see how we came from the morphology and syntax of 6-9 to an interpretation where:

• The moji or uta of the previous sentence ("they") is supposed to be the subject of the conditional (todomareraba), and
• The 4 images (aoyagi, matsu-no-ha, masaki-no-kazura, tori no ato) are targets for the condition: "if they last as long as the bird tracks"… (and not, as I'd expect from the isolated clause: "if the bird tracks last long"…)

I've checked the modern translations/explanations in Iwanami and Shogakukan, and also the Portuguese one by Wakisaka, and they all agree on this point; e.g. Shogakukan:

この歌が青柳の糸の絶えぬごとく[…]鳥の跡の久しく残ることくに、長く後世に伝わりますならば […]

But I'm not used to Classical, so I don't understand where are the ごとくs coming from…

TL;DR: In the first citation above, why does 鳥の跡久しくとどまれらば translate to "if the poems last as long as the bird tracks" and not just "if the bird tracks last long"?

Answer 1

I had forgotten to post it, but here's an answer given to me independently by Matt Treyvauld and by my bungo teacher, Dr Junko Ota from São Paulo University.

The four images above aren't syntactically connected to anything else; they're kind of "floating" in the text, as purely evocative illustrations. This effect may be approximated with modern punctuation:

Should these characters—threads of the green willow!—never cease, and—needles of the pine!—never scatter, and—vines of the jasmine!—reach long, and—tracks of Cangjie's bird!—perpetuate unto posterity…

Because the imagistic clauses are free-floating, the verb phrases of lines 6–10 are all modifying the implied subject kono Uta no Moji.

Answer 2

I'm not really for the suggested translation, so I'm going to translate it myself.

1. Although other people and I will be gone,
2. the events in the songs will remain.
3. Even if time shifts and things leave,
4. (even if) delights and sorrows come and go,
5. (how could I deny) that the characters of this song will be there
6. If the threads of green willows never disconnected,
7. (if) the pine needles never scattered,
8. (if) the masaki-no-kazura vine reached long,
9. (if) the birdtracks were retained for long,

10-13. The people who know the designs of songs and obtain the hearts of words WOULD NOT look up to the high ages as if to look at the moon in the misty sky and yearn for this day