Shavuos VII - why do we say it is the Time of the Giving of the Torah

We all know that Shavuos is the Time of the Giving of the Torah. However such a designation is problematic

1) There is no reference in the Torah that Shavuos is connected with the Giving of the Torah. It is specifically referred to as a harvest festival (Devarim 16:10/Devarim 16:16).
2) Magen Avraham (#494) asks why we say that Shavuos is the time of the Giving of the Torah.
3) Shabbos (86b) has a debate between the Chachomim and Rav Yossi as to whether the Torah was given on the 6th or 7th of Sivan.
4) Rosh HaShanna (6b): Says that Shavuos is sometimes on the 5th of Sivan, sometimes on the 6th and sometimes on the 7th.
5) Rivash (#96) states that there was no fixed link between Shavuos and the Giving of the Torah until the calendar was fixed by Hillel II.
6) Ritva (Shabbos 87b) also asks this question.

Rabbi Yosef Levinson discusses this in a column of De'ah v'dibur

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Chachomim contend that the Torah was given on the sixth day of Sivan, while R' Yosi argues that Matan Torah occurred on the seventh of Sivan. Since the halacha follows R' Yosi's view (Yoreh Deah 196:11) how, asks Mogen Avrohom, can we declare on Shavuos that it is zeman matan Toraseinu (Mogen Avrohom introduction to siman 494)?

Furthermore, we know that the departure of the Jewish People from Egypt took place on a Thursday (Shabbos 87b), while the Revelation took place seven weeks later, on Shabbos (Ibid. 86b). A calculation of the number of days between that Thursday night, which would have been the beginning of the Omer period, and the Shabbos when the Torah was received, comes to fifty-one days. Inasmuch as the Chachomim concur with R' Yosi on this issue, we can conclude that the Torah was given on the fifty-first day of the Omer. Yet Shavuos is celebrated on the fiftieth day after the Omer was brought (Mogen Avrohom, ibid.; see also Rivash no. 96; Maharsha, Avoda Zora 3a; Tzelach Shabbos 88a and Pesochim 68b; Chok Yaakov 430:2 and 494:1).

The Maharal answers (Maharal, Tiferes Yisroel Ch. 27 p. 83a) that although we did not receive the Torah until the fifty-first day of the Omer which, according to R' Yosi, was the seventh day of Sivan, nevertheless Hashem had been prepared to present the Torah to us on the fiftieth day. It was only due to Moshe Rabbenu adding an extra day of preparation that the Revelation was postponed to the following day.

Thus, it is the fiftieth day that we celebrate since Hashem designated that day for Matan Torah. Thus, on Shavuos we declare Zeman Matan Toraseinu, the time of the giving of the Torah. We focus on Hashem's gift to us. Hashem was already prepared on that day to give us His kli chemdoh, demonstrating His deep love for the Jewish Nation, which was thus already fully evident on the sixth of Sivan.

Rabbi Mordechai Kornfeld also has an extensive discussion of this issue