Why did Hashem add the words "When you go to return to Egypt"?
Rashi: Hashem was instructing him that a condition of his Sh'lichus was to go to Egypt staunch in his conviction that he would carry out all the wonders that he had to perform without the least fear of the mighty Pharaoh.
Seforno: Now and whenever Moshe would later return from outside the town of Egypt 1 to speak to Pharaoh, he was obligated to review all the wonders, 2 to make sure that he performs them correctly and in the right order.
Which wonders is the Pasuk referring to, seeing as Moshe did not perform those mentioned earlier before Pharaoh, but before Yisrael ... and which wonders was Moshe 'holding in his hand'?
Ramban #2: Hashem is actually commanding Moshe to perform the three wonders that he would perform in front of the elders of Yisrael in front Pharaoh as well. 2
Midrash Rabah: This is referring to the ten plagues, that were written on Moshe's staff, in the form of the acronym D'tzach, A'dash be'Achav.
Moshe was 'holding them in his hand' in the sense that Hashem had taught them to him in advance. The Ramban also explains "in your hand" to mean that Hashem had appointed Moshe as the sole perpetrator of the miracles that He was about to bring about on Pharaoh. This explains why He warned Pharaoh now, before the act, as well as briefing him at the time that each plague fell due.
Which he did even though the Torah does not record it.
And why did Hashem say "va'Asisem Lifnei Pharaoh, as if it was an independent statement (rather than 'La'asosam Lifnei Pharaoh' - as the conclusion of the previous statement)?
Seforno: If Moshe would fulfill Hashem's command to the letter, then he would 'do them before Pharaoh' successfully - but not if he changed even the smallest detail. 1
As we find by Mei Merivah, when Moshe struck the rock instead of speaking to it. And it is for the same reason that regarding the fulfillment of the Mitzvos, Hashem warned against adding to the Mitzvos or subtracting from them (Seforno).
What are the ramifications of the phrase "va'Ani Achazeik es Libo ... "?
Ramban: Hashem was instructing Moshe here to fulfill every detail of the wonders that he was about to bring on Pharaoh to the letter, and not to become frustrated when Hashem hardened Pharaoh's heart and he obstinately refused to submit.
Rashi (Va'eira) 1 and Seforno: Under normal circumstances, due to the force of the Makos, he would have sent Yisrael away immediately - not because he submitted to Hakadosh-Baruch-Hu, but because he was afraid of the Makos. 2
Why did Hashem harden Pharaoh's heart, and how could he then continue to punish him?