Your plan only solves half the problem. The second answer of the Gemara wants to improve on the first answer, and not be forced to say that the Simanim are just for the different times of the Mishmar. If we say that they are all at the end then we have a reason for both the first Siman (end of time for reciting the night Shema) and for the last Siman (beginning of morning Shema in a dark house). Therefore we choose to say the end Siman.

Dov Zupnik

Here is what the Kollel answered to a similar question. Please let us know if this is not enough.

Best wishes,

Kollel Iyun Hadaf

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Berachos 003: The end of the Ashmuros

yehoshua rabinowitz asked:

why does the gemaras say that that R' Eliezer is really talking about the end of

the moshmarios and for a person in a dark house, you could answer just as well

that were talking about the begining of the mishmaros and someone is in a dark

house so he can't tell if 3 stars are out yet so when he hears the donkey

braying he'll know to get up and say krias shema!

I love the site and you people are great marbeizi toreah

yehoshua rabinowitz, b.h., l.a.

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The Kollel replies:

Your plan only solves half the problem. The second answer of the Gemara wants to improve on the first answer, and not be forced to say that the Simanim are just for the different times of the Mishmar. If we say that they are all at the end then we have a reason for both the first Siman (end of time for reciting the night Shema) and for the last Siman (beginning of morning Shema in a dark house). Therefore we choose to say the end Siman.

Dov Zupnik