Sing

There are three times the Torah uses the expression עצרת regarding a holiday; twice regarding שמיני עצרת and once explaining שביעי של פסח.  In each instance the Targum Yonason says something different.  The word עצרת remains constant, it means (in his view) gathering; what changes is what you gather for.
In parshas Pinchas (29:35) on the possuk בַּיּוֹם֙ הַשְּׁמִינִ֔י עֲצֶ֖רֶת תִּהְיֶ֣ה לָכֶ֑ם, he says ביומא תמינאה כנישין תהוון בחדוא מן מט(יל){לי}כון לבתיכון כנישת חדוא ויומא טבא ואירוע קדיש תהוי לכון, you gather to your houses in happiness.  In Emor (23:36) the possuk says הַשְּׁמִינִ֡י מִקְרָא־קֹ֩דֶשׁ֩ יִהְיֶ֨ה לָכֶ֜ם וְהִקְרַבְתֶּ֨ם אִשֶּׁ֤ה לַֽי״י֙ עֲצֶ֣רֶת הִ֔וא and there the Targum Yonason says כנישין תהון לצלאה קדם ייי על מיטרא, you gather to pray for rain.  What prompts him to change his explanation of what we gather for in these two pessukim?
Regarding the last day of Pesach, the possuk in Reah (16:8) says  וּבַיּ֣וֹם הַשְּׁבִיעִ֗י עֲצֶ֙רֶת֙ לַי״י֣ אלקיך and there the Targum Yonason explains וביומא שביעאה ההוא תהון כנישין בתושבחא קדם ייי אלקכון, we gather to sing.  I understand why he says this only in the context of שביעי של פסח, not Shemini Atzeret, for it is only on שביעי של פסח that the holiday commemorates an act of singing.
The Sforno on this possuk says וביום השביעי עצרת – נעצרו בו ישראל יחדיו לעבודת האל יתברך, ושרו לו שירה בשביעי של חג המצות, לפיכך נקדש אותו היום.  He also seems to be translating that word עצרת as gathering but he explains it not in the future tense, that we should gather, but the possuk is giving the reason for the day being a holy day is because we gathered, to sing.  [Well I lied about him translating עצרת as a gathering , he really translates it as holding back, but it means Klal Yisroel remained near the sea to sing shirah, see the Sforno in Emor, but the point is the same.]  The Sukkot Dovid points out that we see the Sforno doesn't say the miracle of the splitting of the sea, it is the fact that we sang shirah that makes the day a holy day.  Another miracle wouldn't have made a holy day, not every day of miracles is a holy day, it is only the fact that we internalized the holiday through our shirah that made it a holy day.  In light of this Sforno that the whole source of the holiness of the day is our saying shirah we understand very well the Targum Yonason that the avodas hayom is to sing shirah.
Once I'm mentioning שמיני עצרת and parshas Reah, I was wondering why does שמיני עצרת not make an appearance there?