When exactly, was Yitzchak born?
Rashi #1 (citing Midrash Rabah): There are two opinions as to whether he was born after nine months or after seven. Both opinions agree however that he was born on Pesach - Rosh Hashanah 10b.
Rashi #2 (citing Midrash Tanchuma): Based on the word "Oso" 1 , the Tanchuma explains that he was born exactly one year later (on Pesach).
See Sifsei Chachamim.
What does "la'Mo'ed" mean?
Rashi #1: It means after nine months (the appointed time that a woman gives birth). 1
Rashi #2: It means after seven months (the time to which the angels referred earlier in the Parshah).
Rashi #3: The angel made a scratch on the wall, and exactly one year later, when the sun reached the same point on the wall as it had been when the angels came to see Avraham, that son was born. 2
What is the meaning of 'li'Zekunav"?
Targum Onkelos and Targun Yonasan #1: It means literally 'in his old age'.
Targum Yonasan #2 and Rashi: It is the acronym of 'Ziv Ikunin' (his shape of face), because Yitzchak's face was strikingly similar to that of Avraham.
Why does it need to say that she became pregnant? It suffices to say that she gave birth!