What is the significance of the order in which the Pasuk presents the three groups of soldiers who are obligated to return home?
Torah Temimah, citing Sotah, 44a): It teaches us that according to the laws of Derech Eretz, a man should first build a house and plant a vineyard and then get married.
Why might they otherwise die?
Rashi: Because if they disobey the Kohen's 1 orders to return home, they deserve to die.
Ramban #1 (on Pasuk 8, according to R. Yossi ha'Gelili): The Torah means (not that he will die, but) he thinks that he will die.
Ramban (on Pasuk 8, according to R. Akiva): If , after the Kohen's assurance, he is still afraid - he lacks Bitachon and will not merit the miracle that the Kohen promised.
Seeing as a man who has not married his betrothed is sent home, what is the difference between him and a Chasan, whom the Torah in Devarim 24:5, forbids to go to war for the entire first year after his wedding?
Rashi (in Devarim, 24:5, citing Sotah, 43b)): The Chasan may not leave his house for anything connected with war, neither to provide the army with water and food, nor to prepare the roads (sappers), whereas all those whom the Kohen sends back from the front ... are still required to provide water and food and to prepare the roads.