akuperma, <i> Ha-Shem got upset at us for being too happy when Ha-Shem drowned the Egyptians</i>
No, he did not. On the contrary, the gemora you refer to explicitly says that Hashem completely approved of our celebration and shira, and objected only when the angels joined in. The gemora’s exact words are הוא אינו שש אבל אחרים משיש , “He does not rejoice, but He makes others rejoice”.
May I suggest that the reason for the discrepancy is that when the BHMK stood the emphasis was indeed on the victory, and Ve’al Hanisim dates from that time, but after the churban, when we no longer had the benefits of the victory, that aspect became less important, but we still have the spiritual benefits of the miracle, so that is now the main focus of the celebration.