"Amen" in versions of Kaddish

One of the differences between Ashkenazic and some other liturgies is the text of the Kaddish. In Ashkenazic siddurim, the text reads (I'm using Sefaria)

יִתְגַּדַּל וְיִתְקַדַּשׁ שְׁמֵהּ רַבָּא (אָמֵן)

בְּעָלְ֒מָֽא דִּי־בְרָא כִרְעוּתֵהּ וְיַמְלִיךְ מַלְכוּתֵהּ בְּחַיֵּיכוֹן וּבְיוֹמֵיכוֹן וּבְחַיֵּי דְכָל־בֵּית יִשְׂרָאֵל בַּעֲגָלָא וּבִזְמַן קָרִיב וְאִמְרוּ אָמֵן:

but in other texts (** Sephardic ** and _ Chabad _) we see

**יִתְגַּדַּל וְיִתְקַדַּשׁ שְׁמֵהּ רַבָּא בְּעָלְ֒מָא דִּי בְרָא כִרְעוּתֵהּ וְיַמְלִיךְ מַלְכוּתֵהּ וְיַצְמַח פּוּרְקָנֵהּ וִיקָרֵב (קֵץ) מְשִׁיחֵהּ. בְּחַיֵּיכוֹן וּבְיוֹמֵיכוֹן וּבְחַיֵּי דְכָל בֵּית יִשְׂרָאֵל, בַּעֲגָלָא וּבִזְמַן קָרִיב וְאִמְרוּ אָמֵן **

or

_ יִתְגַּדַּֽל וְיִתְקַדַּֽשׁ שְׁמֵהּ רַבָּֽא. אמן בְּעָלְמָֽא דִּי בְרָֽא כִרְעוּתֵהּ וְיַמְלִֽיךְ מַלְכוּתֵֽהּ, וְיַצְמַח פּוּרְקָנֵהּ וִיקָרֵב מְשִׁיחֵהּ. אמן בְּחַיֵּיכוֹן וּבְיוֹמֵיכוֹן וּבְחַיֵּֽי דְכָל־בֵּֽית יִשְׂרָאֵֽל, בַּעֲגָלָא וּבִזְמַֽן קָרִיב וְאִמְרֽוּ אָמֵֽן:_

A first question:

In the Sepharic version, there is no "amen" after Raba (which makes more sense to me than putting one in there) but there is in the Chabad version (or is it just missing from the Sepahrdic trascription, but it is said). Should there be an "amen" there?

A bigger question:

In the Chabad version (at least) there is an "amen" after meshichei. This version includes 2 phrases after "malchutei" as additional statements about our future hopes, and then we answer Amen.

But if they are viable hopes that we agree to through our "amen" why is there no Amen in the Ashkenazic version after "malchutei"?

Is there something in the extra 2 phrases that demands an "Amen" whereas the single aspiration doesn't?