Seeing as Adam was already sentenced to die, why did he receive the additional punishments mentioned here?
Seforno: The death-sentence was for eating from the fruit of the Eitz ha'Da'as, as Hashem warned him, but not for listening to Chavah and accepting the Lashon ha'Ra that she spoke about Hashem, to whom she attributed the Midah of jealousy and said that He lied. For that, he was receiving the extra punishments.
How was the earth cursed?
Rashi: It will produce cursed creatures, such as flies, beetles and ants.
Lev Eliyahu (Bereishis p.25): No food ever spoiled until Adam sinned.
Why was the earth punished together with Adam?
Rashi #1: It is a Mashal to someone who went off the path, and people cursed the breasts from which he suckled. 1
Rashi #2: The earth was punished because it disobeyed Hashem's orders - to produce trees whose wood had the same taste as its fruit. 2
Seforno: It was not the ground that was being cursed, but Adam - inasmuch as the ground would no longer yield its produce unless he tilled it (which was not the case before the sin).
Targum Yonasan: The earth was punished for not rebuking Adam, to prevent him from eating from the forbidden fruit.
Why was his punishment attributed to obeying his wife?
Ohr Yechezkel Midos p.290: The Torah teaches that even though he thought that he is doing a Mitzvah (obeying his wife), it was the most severe Aveirah. All the more so we, who are not at his level (angels wanted to sing Shirah to him), if we do not know evil, we can think that our bad deeds are Mitzvos!
Chachmah u'Musar (2 p.279): The ability that leads to sin 1 is worse than sin itself, for it contains many sins. One must uproot it, even a Chacham like Adam. This shows how much one must distance from Heter, lest he come to Isur!
Lev Eliyahu (Bereishis p. 186): He leaned to listen to his wife, without contemplating whether or not this is Hashem's will. The Midrash (Bereishis Rabah 19) says that he said "I ate, and I will eat" - she has power to entice, and I cannot pass the test.
Why were all generations punished for Adam's sin?
Lev Eliyahu (Bereishis p.38): Before the sin, Adam's body was clear, and he benefited from the Shechinah in Gan Eden. Through the sin, his entire body became sick and physical. He cannot benefit from the radiance of the Shechinah, and has no place in Gan Eden, until the Ru'ach separates from the body. It is not a fine - it is reality. He ruined the world. Refer to 3:22:1:2*.
רש"י: מעלה לך דברים ארורים: מנין לפרש כך?
גור אריה: רש"י התקשה שלא כתוב בפסוק במה ארורה האדמה, ולכן פירש שתהיה מעלה דברים ארורים.
רש"י: משל ליוצא לתרבות רעה וכו': ברש"י נראה שזה פרוש אחד, ואילו במדרש מבואר שאלו שני פרושים?
גור אריה: מוסכם שהקללה היא שהאדמה תוציא דברים ארורים, אלא שלפרוש ראשון זו קללה לאדם, ולפרוש השני נתקללה האדמה עצמה מחמת המשל שמביא רש"י, ורש"י נקט כפרוש השני.
רש"י: משל ליוצא לתרבות רעה וכו': קשה שלעיל (א,יא) כתב רש"י שהאדמה התקללה מחמת החטא שלה שלא הוציאה עץ שטעמו שווה לפריו?