1) I missed posting this before Shabbos: "V'haya bayom ha'shishi" -- ain v'haya elah lashon simcha. TGIF is right there in the Torah!
2) The Kozhiglover explains that Rashi's interpretation of the future tense of "yashir" as meaning, "alah b'libo," that the feeling/thought of shirah arose in Moshe's heart/mind before he spoke, goes hand in hand with Chazal's interpretation of the future tense being a hint to techiyas hameisim. Every action is preceded by thought -- what is unique about shirah? The answer is that thought is not just a necessary precursor to shirah, but it is the very definition of shirah -- shirah is that which is in the heart, it is a feeling that words are almost inadequate to capture. Shirah is the highest level of dveikus, a level at which a physical body, a physical mouth, is just an intrusion. This level of connectedness to Hashem is something reserved for the time of ultimate redemption. It is only because Moshe was able to sense the ultimate geulah, he felt the "remez l'techiyas hameisim," that "alah b'libo" to sing shiras ha'yam.
The Midrash comments that Moshe sinned with the word "az" when he complained that "Mei'Az basi el Pharaoh l'dabeir b'shemecha hei'ra la'am hazeh," that from the moment he asked Pharaoh to free Bnei Yisrael things had gotten worse for them, and he corrected his error with the word "az" by singing "Az yashir." The question that much ink has been spilled trying to answer (which we discussed once before as well) is why the fact that Moshe used the little word "az" in his complaint to Hashem has any significance -- he used lots of other words too, and we don't find them in the shirah? Those that follow in the footsteps of the Sochotchover (the Kozhiglover has a few pieces on this, it is also in a derasha in She'eiris Menachem) explain that there is a thematic connection that the Midrash is hinting at. Moshe's use of the word "az," then, at that moment, implies that he felt there was a timing problem. Moshe's thought that achieving geulah after only 210 years was an unrealistic goal; maybe it would be better to wait. After all, things were getting worse, not better; Pharoah was increasing the burden on Bnei Yisrael, not giving an inch. Moshe lacked the perspective to see how the suffering of the present moment would in fact lead directly to an immediate geulah; he had no sense of what the future would soon bring. It was that lack of perspective that Moshe corrected at Yam Suf. "Az yashir" -- in the future tense, fully aware not only of the redemption of the moment, but of "remez l'techiyas ha'meisim," future geulah as well, which in turn gave rise to dveikus in the present.