Remnants of Repentance
It is stated, "repentance was created before the universe itself; that repentance was a pre-condition of Creation". If so, how is it that G-d is so intensely angered at the Jews for the sin of the golden calf? Or was that just for "show"? Furthermore, how is it that G-d, in response to Moshe's subsequent impatience with the Jewish people [that led him to strike the stone to get water instead of speaking to the stone as commanded by G-d], forbids him from entering the land?
You are raising a very good question: If repentance is so important as to precede Creation, and if it is so essential that it was actually a pre-condition for Creation, why does it seem not to have helped in the cases you quote?
True, our Sages taught that repentance was one of the seven creations created pre-Creation (Pesachim 54a). And as you mention, one explanation for this is that G-d foresaw that mankind would sin, calling into the question the very justification of Creation. He therefore made an entity called teshuva — repentance — to ensure the perpetuation of Existence. This is the meaning of the teaching of our Sages: "The rabbis asked Wisdom, What is the punishment due to a sinner? Wisdom replied, he should be pursued by evil. The rabbis then asked G-d [the same question.] He replied, let him repent and he'll be forgiven" ( J.T., Maccot 2:6).
Still, when pondering the power of repentance, it is important to consider the severity of the transgression as well. Clearly, relatively minor offences can be more easily mitigated by regret than severe ones. Yet, regarding some acts, no amount of repentance can rectify the ramifications of the wrong, and punishment is required despite the repentance.
This was the case regarding the sin of the golden calf. Up until that event, the Jewish people had witnessed such fantastic and open miracles at the hand of G-d such as the plagues, the splitting of the sea and the events at Mount Sinai that it was an abomination in the eyes of G-d that they should lapse into worshipping the calf, which itself is a transgression liable of the death penalty, made all the worse considering what they had witnessed. For this reason, 3,000 people were killed for actively worshipping the calf despite being warned and observed by witnesses.
Furthermore, the sin of the calf is compared to adultery — another of the cardinal sins subject to capital punishment. Thus Moshe broke the Tablets, compared to a ketuba between G-d and the Jewish people, before they were given, in order to diminish the infidelity of His faithless bride. Similarly, Rashi explains that the reason Moses ground the calf, mixed it with water and gave the Jews to drink the potion was in order that, in the absence of witnesses, the Jews would be tested much as a woman suspected of faithlessnesswas tested with sotah water.
Given the idolatrous and adulterous nature of the sin of the golden calf and its resultant desecration of G-d's name, it is clear why G-d was so angry, and why, even though the Jewish people repented (Avoda Zara 4b), their sin was not fully erased. This can be further understood when considering the third cardinal sin – murder. No matter how much a person would repent such an act, teshuva wouldn't forestall punishment to exempt him from the death penalty. On the contrary, teshuva only assures that the death will atone — but the penalty is still administered.
Perhaps this can help us understand your question regarding Moshe. Relative to Moshe's extremely lofty spiritual position, his becoming angry with the Jewish people, calling them "rebels" and deviating from G-d's command to speak to the rock by hitting it instead, was tantamount to disbelief in G-d and to chillul Hashem – a desecration of G-d's name. This is implied in the verse, "Because you did not believe in me, to sanctify me in the eyes of Israel, therefore you shall not bring them into the Land" (Num. 20:12). In fact, chillul Hashem is considered one of the gravest offences, and is so described by Rambam in his Laws of Teshuva. Even though Moshe surely did the highest form of teshuva m'ahava – repentance out of love for G-d, not fear of punishment — this apparently was not enough to fully atone for the (relative) severity of his act, and perhaps the decree that he be barred from entering the Land of Israel was needed for a complete atonement.
That being said, the words of Midrash Yalkut Shimoni (Parshat V'etchanan) are very illuminating. In discussing Moshe's 515 prayers to repeal the decree, the Midrash portrays in great detail the dialogue between Moshe and G-d in the prophet's plea to enter the Land. After hearing Moshe's very compelling arguments, G-d finally replies that either he'll have to die outside the land or the Jewish people will have to die. This enigmatic response may be explained as follows:
If Moshe and Aharon would enter the Land, the Temple they would build would be eternal. However, G-d saw that the Jewish people would eventually greatly sin and be liable for destruction. The destruction of the Temple was thus necessary as a means of arousing Jews to repent. Had it not been vulnerable to destruction, harsher measures against Jewry would have been required. For this reason Moshe was prevented from entering the Land.
According to this, Moshe's teshuva effected complete atonement for himself. Still, he was barred from entering Israel for the ultimate good of the Jewish people. When Moshe understood this, he made the ultimate sacrifice for the people he so loved and prayed no more to repeal the decree.