Why is Yismach Moshe in the Shabbat Amidah?

Question

On Shabbat morning, the fourth Amidah blessing includes Yismach Moshe:

"Moshe rejoiced in the gift of his portion: That You called him a faithful servant. A crown of splendor You placed on his head when he stood before you on Mount Sinai. He brought down two stone tablets in his hand, on which is inscribed the observance of the Sabbath. So it is written in Your Torah..." [then the Veshameru]. [ArtScroll translation]

(1) It is odd. Why is it there? What would be lost if it were removed and we went right to Veshemeru, which is a Torah quote [Ex. 31:16-17] that is more illuminating?

(2) Who composed it and when was it introduced in the siddur?

Background note: Rashi wanted it out, but he was obviously ignored. In Sefer Manhig, Hilkhot Shabbat #20, p. 150, we see: Rashi would not say Yismach Moshe, but would say “You chose us.” He would say: “Hashem, our God, gave us Sabbaths for rest.” Because he didn’t know what the connection was between Shabbat and Yismach Moshe!.. But [his grandson] Rabbeinu Tam restored the matter to its original glory.

Answer 1

See the Kli Yakar on Shmoth 31:13, that at some level Shabbat belongs to Moshe since he chose it in Egypt as the day of rest for us slaves.

ואתה דבר אל בני ישראל לאמר אך את שבתותי תשמרו. הוסיף כאן מלת ואתה לפי שיום השבת ראוי ליחס בעצם וראשונה אל משה כי הוא בחר במצרים ביום השבת כדאיתא במדרש (שמו"ר א כח) ובנוסח התפלה ישמח משה במתנת חלקו כו'.‏

So maybe, since the Egyptians let us sleep at night, Moshes "gift" was only noticeable in the morning.

Answer 2

There is a thematic thread that courses through the Shabbos prayers reflecting the ascent in kedusha over the course of Shabbos Kodesh. From a kabbalistic perspective, as Shabbos progresses we move to ever-higher levels of kedusha with Friday night being the lowest level as we transition from chol into kodesh; Shabbos morning being the next level of holiness; and the end of Shabbos being a culmination, - a crescendo, really - as we arrive at the highest level of kedusha.

This is paralleled by the idea that Shabbos as a whole is the marriage ceremony between the Jewish people and God, as it were. Friday night corresponds to the "kiddushin" and thus the Friday night prayers highlight this with the paragraph of "Atah kidashta". Shabbos morning is the simchas chosson v'kallah, reflected in the paragraph of "yismach Moshe" highlighting the joyous nature of our relationship; finally, the mincha services allude to the sacred union - yichud between husband and wife with the "Atah echad" passage in the amidah.

For more in-depth examinations of these ideas, one can refer to Sefer Nesivos Shalom, Be'er Mayim Chaim, and other sources.