This is what I gathered:
_1st thing that happened after Seth lost his memories: Osiris tried to make Seth remember and almost destroyed him, so he secretly made the desert bigger in order to keep Seth alive while he kept trying to force him to remember (according to chap. 198).
_2nd thing that happened: when that didn't work, Osiris went to Hermopolis in search for the answer, and killed all the humans there when he was unable to find it. (according to chap. 198).
_3rd thing that happened: Osiris decided he needed to become King of Egypt to have enough authority to be received by Ra, to whom he wanted to ask at first how to restore Seth's pre-god memories, and later on how to have a child with him (despite both of them being male).
-4th thing that happened: Osiris figured out that to become king of Egypt, he needed to use all his siblings.To become king, Osiris needed a stable Egypt, which means he needed at least a few humans to survive, which means he needed to keep those humans well-fed, and here's where Isis came in: to make the sand fertile enough for agriculture, they needed the Nile river to flood, and Isis was the only one who could do that, so Osiris married her to guarantee that his kingdom would have that (Ra would not accept a meeting with an incompetent king wannabe). To make Seth want to spend time in Egypt (rather than keep wondering around), Osiris tried to make Seth feel useful to Egypt by naming him the god of war, with the half-truth that "they needed someone to protect Egypt from invaders," and then let Seth marry Nephthys because Nephthys as goddess of peace would obviously stay where the humans were, in Egypt, and her relationship with Seth turned her into was the only one who could make Seth want to come back to Egypt from each time he left (be it to the battlefield or just because he was fed up with his siblings).
-5th thing that happened: After Seth killed Osiris, Isis decided she needed to have Osiris' child to get revenge, and got herself impregnated by Osiris during the brief time in which she managed to temporarily resurrect him. Osiris' reason to help her is probably related to what he says in chapter 34, that he felt he "owed it to Isis to at least let her have a chance at revenge," since that method of revenge then gets explained by Isis in chapter 46, where Isis says she needed a son to get her revenge for her because she was unable to become King herself due to the curse (the one that she had put on herself before, along with her siblings, that none of the 4 siblings would get what they wanted). Her son, though, would not have that curse hindering him, and would have a strong claim due to being Osiris' son.
Forgot to add the chapters for these:
_3rd thing that happened: Osiris deciding that he needs to become king in order to force Ra to tell him a solution (is mentioned in chap. 198 as well)
_4th thing that happened: Osiris decides that to be king, he needs a kingdom, and to have a kingdom, he needs to feed at least *some* humans in Egypt (a number big enough to have a kingdom to be the king of), so he marries Isis, so that he can put her in charge of Egypt's food supplies: the connection between Isis' power over the Nile river and Egyot's food supplies is hinted first in chap 1, where we hear Isis promised that "for (Osiris), she would never let the Nile river run dry." This is significant given how in chapter 164 Seth explains that Egypt's food supply depends on the Nile river flooding (in chapter 184 Seth explains that "without water, no plants can grow in sand, and then in chap 163: "Ever since Isis took over the food supplies, the humans starving was a thing of the past)." Seth's and Nephthys' roles are explained in chapter 35: Seth being named "god of war" so that he would feel tied to Egypt, and Nephthys' role as Seth's leash (the leash that would keep Seth coming back to Egypt... and thus, coming back to Osiris), which is hinted at in chapter 35 in the phrase: "Osiris allowed Seth to marry Nephthys, to contain the sand within the realm of peace".
-5th thing that happened: in chapter 8, Osiris "explains" his temporary resurrection by Isis, which allowed for Horus to be conceived.
Wow the possibilities are a lot abt what could've happened and all of these make sense so Much I cant disregard or call them a stretch. Osiris wresting the rule from ra surely must have been part of his grand schemes of obtaining Seth... and omg you are so right. I could be that he really didn't want a kid with isis but had to give in because isis was a valuable pawn to him and he had no choice but allow horus to be conceived for her revenge.. I kinda believe the reason he made nephyths marry seth was 1. Like u said, tie him to egypt and him and 2. He manipulated and tempered broke n remade seth so many times enough to enlarge dessert n engulf all of egypt that he NEEDED nephyths to ensure seth doesn't break and lose sanity completely she was there to heal and piece his soul back together again and again and forcefully shove harmony n peace upon him to keep him blinded n stable. Like a comatose patient on life support
One thing i find fascinating is ra knew what would come of egypt. She kept making negative remarks abt gods who started living as humans. Its true gods arent mortals they're complete omnipresent beings themselves who dont need family but yet these gods married, had kids and started living like humans which is why we see the current state of egypt. On surface its hardly a ripple but the undercurrents are turbulent social order has perished the country is dying invaders at heels gender ratio in imbalance, starvation famine and all sorts of horrors occurring against women n children. To think the relationship between these siblings were the fundamental foundation for egypt and After HTE's death everything has descended to chaos and unrest. Gods need a purpose to ascend and its hinted by murals that Isis and nepthys ascended for their brother. Which could mean either seth or HTE. They must have had a really close relationship esp isis if she really became goddess of magic for HTE and in ennead too young horus grew up reading abt the guardian god seth because isis had not left a single record abt seths glorious deeds military strategies his war achievements and all his stories. She maintained it in her temple as if she were proud of him until seth razed it to the ground.
Was re reading some chapters and one particular dialogue by horus really struck me.. he mentions that osiris didn't/ never planned to have a child with isis... now what might be the reason behind it. Ofc he was obsessed with seth but he was willing to go far as to play the house n pretend family with isis and the siblings. Unless HTE is true and he knew that present isis wanted to bring HTE back he really had no reason to avoid impregnating her esp when he was desperate to do the same to Seth and babytrap him