rashash vs aruch hashulchan on placement of nacheim

The Yerushalmi writes that the bracha of nacheim should be recited in avodah, similar to the way we recite ya'aleh v'yavo in avodah.  Yet that's not what we do -- we stick it in bonei yerushalayim.  There is no source in the Bavli that says to do that, so why do we disregard the Yerushalmi?

RI"F writes that our practice is based on the Bavli's din that one is permitted to add a bakasha to a bracha so long as it is in keeping with the bracha's theme.  For example, if you know someone is sick, you can add a bakasha for them to be healed into refa'einu.  We therefore add our bakasha for nechamas Yerushalayim to the bracha that deals with binyan Yerushalayim.

Aruch haShulchan is not satisfied with this alone.  He points out that the Yerushalmi may be l'shitaso.  The Yerushalmi's nusach ha'tefilah incorporates es tzemach David into bonei yerushalayim as one bracha. Therefore, since the theme of nacheim does not relate to es tzemach David, it cannot be incorporated into the bracha of bonei yerushalamim/es tzemach as it only fits half the topic.  The Yerushalmi had no choice other than to put it in avodah.  According to the Bavli that es tzemach is its own bracha and bonei yerushalayim is its own bracha, nacheim fits perfectly into bonei yerushalayim, and that is why we put in there.

Even though we reject the Yerushalmi, the Rashash in Taanis (30) comes up with a chiddush l'halacha based on it that is implicitly at odds with this Aruch haShulachan.  Some background: S.A. writes (294:4) that if you omit something from tefilah that you are not required to repeat for, once you finish the bracha you cannot go back.  R' Akiva Eiger adds that you don't even have to finish the bracha -- once you say "baruch ata Hashem..." and say the shem Hashem, you can't go back.  For example, if you forgot havdalah on motzei Shabbos in the bracha of ata chonein, once you said "baruch ata Hashem" to end the bracha, you just go on.  The MG"A (114:8) adds that you cannot even add the missed words in between the bracha you finished and the next bracha either -- i.e. it is not just a matter of avoiding wasting shem Hashem by going back.

Rashash argues that nacheim is different.  If you miss saying it, you can stick it in between the brachos.  Since according to the Yerushalmi es tzemach David and bonei yerushalayim is one bracha, the placement of nacheim where we have it must assume that there is a connection between nacheim and the themes of both brachos (not like Ah"S who says Ylmi l'shitaso would not hold of where we place it!) Therefore, even if you finished the bracha of bonei yerushalayim, you are still holding in the same topic by es tzemach david and may add in nacheim.  (There are a few other wrinkles that he adds to bolster the point, but this is the main argument.)

Aside from the chiddush l'halacha, there is an important idea in hashkafa as well.  According to Rashash, the theme of binyan yerushalayim that we hope and pray for in nacheim does not stand by itself -- it is linked to es tzemach david, to geulah and mashiach.  Even though we may even be able to stand in Yerushalayim on 9 Av under an Israeli flag, we still must recite the tefilah, as the redemption of Yerushalayim remains incomplete so long as we lack full geulah.