Is it possible for one creature to change into another?

Iyun Yaakov, Ben Yehoyada: These changes happen after its death. Either after it decomposes it is reincarnated in this fashion (Iyun Yakov) or its maggots take on a form that appears like a bat etc. (Ben Yehoyada)

How is this a suitable punishment?

Tosfos: There is a Mitzvah to straighten up like a snake after bowing. Since he did not fulfill this, he is punished measure for measure and his spine becomes a snake.

Maharsha, Maharal: If a person does not bow but rather he stands erect, his spine is acting in the way of the original snake who walked with spine erect in a proud way and did not bow to Hash-m's will. Therefore, it is fitting for him to be punished through his spine becoming a snake. If a person does bow in Modim, he shows the difference between him and the snake.

Tosfos, citing 'others': Chazal teach that a person is resurrected from the 'Luz' bone in his spine that never decomposes. When he doesn't bow in Modim, a person is punished that hs Luz bone turns into a snake so that he will not be resurrected in the World to Come. Why a snake? Anaf Yosef explains that just as the snake 'will not be healed [from its curse] even in the World to Come' (Bereishis Rabah 20:5), so, too this person will not 'be healed' from man's curse of death.

Why does this metamorphosis take 7 years?

Etz Yosef: The gestation time of a snake is 7 years.

Why did King Chizkiyahu merit to be buried next to Kings David and Shlomo, more than other kings of the House of David?

Maharsha: Shevna, Chizkiyahu's Royal steward, had tried to take away the honor of the kingdom for himself, by digging himself a grave in the place where Chizkiyahu was supposed to be buried (see Sanhedrin Daf 26). When Shevna was ultimately killed and not buried there, they did not want to use that grave for Chizkiyahu and in respect for his greatness, they buried him instead near the graves of David and Shlomo.

Iyun Yaakov: Other kings were buried next to their fathers, but Chizkiyahu's father had not been buried the burial place of kings, as (with the approval of the Chachamim) Chizkiyahu had dragged his evil father (King Achaz)'s bones around in a rope stretcher (see Mishnah Pesachim Perek 4). Therefore, he had to be buried near the best of the family.

Etz Yosef, Ein Eliyahu, Ben Yehoyada: Chizkiyahu himself studied and brought the nation to study Torah even more than Moshe and Yehoshua. Ein Eliyahu adds that for this reason he was even worthy to become Moshiach (Sanhedrin 20b). Thus Chizkiyahu was similar to David and Shlomo in the amount of Torah that he spread and taught (Ben Yehoyada).

Why was only King Asa's coffin filled with spices, which is something not mentioned about other kings?

Maharsha: Perhaps it is because he suffered from foot ailments for the last two years of his life. (Presumably, foot ailments cause a bad smell. MDC)

Iyun Yaakov: In his illness he had relied on doctors rather than on Hash-m. Therefore, after he died he needed to be embalmed by doctors, something that other righteous kings did not need (as is clear from Bereishis Rabah 100:3).

What was the essence of their false claims against Yirmeyah?

Tosfos: They suspected him of having relations with a harlot. Yirmeyah was a Kohen and prohibited to a harlot.

Ben Yehoyada: The suspicions were not accusing Yirmeyah himself of sinning so blatantly. Rather, R. Elazar says that they suspected Yirmeyah's wife of being raped, thereby prohibiting her to Yirmeyah, who was a Kohen. (She would now be in the category of a harlot.) R. Shmuel bar Nachmani says that they suspected that his wife's father had accepted Kidushin for her from another man when she was a child and her father then died. This would mean that Yirmeyah's wife was married to another man and that he was living in an adulterous relationship.

Maharal: They acknowledged that he had prophecy, but claimed that it came through his attachment to an external power. The meaning of being suspected of relations with a harlot, is that he was attached to a level that was not exclusive to him, like a harlot that is not exclusively his. The meaning of being suspected of relations with a married woman, is that he was attached to a level that was not fitting for him at all.

Why did Yirmeyah curse the people of Anatos in this way?

Shita Mekubetzes citing Rabbeinu Yishayah: Charity would have been capable of appeasing Hash-m's anger against them, as it says (Mishlei 21:14), "A gift in secret will appease wrath". Yirmeyah prayed that their charity should not go to a suitable place, so that they would not have that protection.

Iyun Yaakov: The Gemara in Sota (4b-5a) cites R. Yochanan's teaching - Whoever has relations with a married woman, even if he gives charity in secret, about which it is written, "A gift in secret will appease wrath", he will not escape the punishment of Gehinnom. Therefore, since they had suspected Yirmeyah of this sin, Yirmeyah prayed that, measure for measure, they should be punished in this way.

Rabbeinu Peretz: Since he learned and spread so much Torah, it was not considered to be 'mocking the poor'. Alternatively, they studied more than 4 Amos away from the grave.

Maharsha, Maharal: It was permitted since it was done in his honor.

Maharsha: Perhaps it was a set up as a center to mourn his death (rather than a place of learning).

What is the basis of the dispute about the three durations of the learning done at his grave?

Maharsha: They correspond to the three stages of Aveilus, mourning, given in Moed Katan 27b - three days of crying; seven days of eulogy and 30 days for (refraining from) pressing clothes and cutting hair.

Maharal: The opinion that says three days is because the Gemara states (Shabbos 151b) that the body remains intact for three days after death, but after that, the stomach bursts etc. Since in that learning, it is as if the dead person is also studying with them, that is only the case whilst the body is intact.