Yep, Edelgard is DEFINITELY the elephant in the room. And a big one, too. Which is why it would most likely be better to try and talk to her before the timeskip. Somehow get at least a C support with Edelgard, Claude AND Dimitri by giving them presents or inviting them to tea, and that COULD unlock the gateway to a Golden Ending. Somewhere along the way, Edelgard will have a case of "My God, What Have I Done?" and wonder if she's really doing the right thing after all, all because of the talk you had with her five years ago. In fact, Edelgard would be the hardest to convince, maybe you would have to get a B support with her.
Dimitri would be the second hardest to convince, only because he is suffering from PTSD and God knows what else. But he could possibly be saved in Verdant Wind or Silver Snow is his support level with Byleth is high enough? (He won't survive Crimson Flower simply because he is against Edelgard.) Claude would be the easiest to convice due to being level-headed, and if he gets a high enough support with Byleth, he should appear in Silver Snow, given he can already survive all other routes.
So Byleth gets a high enough support level with Edelgard, Dimitri and Claude before the timeskip, and the route seems to go as normal depending on what house you chose. Here's my idea:
- If Dimitri gets a high enough support level with Byleth in Verdant Wind/Silver Snow:
-- Dimitri will realise that going up against the Imperial Army at Gronder Field is suicide, and will turn to Byleth for help in Silver Snow.
-- Dimitri will stop pursuing Edelgard in Verdant Wind when he gets exhausted, having held back.
-- He will still die in Crimson Flower because Edelgard is too set in her ways, unless Claude gets a high enough support level too.
- If Claude gets a high enough support level with Byleth:
-- He will NOT attack the Blue Lions in Gronder Field. If you talk to him, the Golden Deer become Allies.
-- He cannot be executed in Crimson Flower, because Byleth absolutely won't let Edelgard kill him.
-- He makes an appearance in Silver Snow, instead of going missing forever.
- If Edelgard gets a high enough support level with Byleth:
-- She will take Dimitri's hand and try to atone for her sins when he spares her in Azure Moon.
-- She will be taken prisoner in Silver Snow, because Byleth refuses to kill her.
-- Similar thing happens in Verdand Wind as Silver Snow
The Golden Ending could run like this: Silver Snow: Byleth and his army makes it to the three way battle and is able to stop the House Leaders from killing each other. They fight TWSITD (which makes the House Leaders trust each other more) and try to save Rhea, but she still goes berserk and needs to be defeated too (but not killed). Fodlan is no longer run by the church but Byleth is the new leader, with the three houses having different roles in the leadership. Azure Moon: Dimitri is still mentally unstable, but in the three way battle at Gronder, Claude and the Alliance join Dimitri and Byleth. Eventually, Dimitri tries to make peace with Edelgard, but is forced to fight her anyway. However, Edelgard accepts Dimitri's mercy at the last minute. TWSITD attacks because they are angry that the House Leaders are no longer fighting each other and fear that the Church will reign supreme, but everyone fights them off. Dimitri now rules Fodlan, and Byleth takes over the Church. Verdand Wind: Claude's Army stands up to the Empire. Dimitri tries to pursue Edelgard but fails, and he retreats, having held back. He reluctantly joins Byleth's army. They rise against the Empire and defeat Edelgard, but Byleth refuses to kill her, because their bond with her is too strong. They take her prisoner and she fights for them when they stand up to TWSITD. Rhea's involvement in defeating TWSITD seems to change Edelgard's view of the Church. Fodlan is now ruled by Byleth and Claude and racism is abolished and the borders are opened. Crimson Flower: Edelgard defeats the Alliance, but Byleth absolutely refuses to let Claude and the Golden Deer die and have the Alliance fight for the Empire instead. They then attack the Kingdom and defeat Dimitri, but Dimitri is spared when Byleth stops him and the Blue Lions from turning into demonic beasts and heals everyone, Edelgard wonders what the hell Byleth is doing but memories of their childhood friendship resurface, and Byleth convices her that killing her old friends in order to fulfil ideals is wrong, and Dimitri finds out that Edelgard is not responsible for the Tragedy of Duscur, etc. so the Kingdom reluctantly joins the Empire. Rhea goes berserk and turns into a dragon, but Byleth is able to calm her down, much to Edelgard's shock, but Edelgard learns that the Church is not as bad as TWSITD, who are responsible for a bunch of the things that happened. They fight TWSITD and win. Fodlan is united by the Empire under Edelgard's rule and the Church, while it still stands, no longer has a part in ruling Fodlan, and Edelgard abolishes the crest system altogether, making Crests nothing but a tool in battle/life.
This is only a rough idea of what could happen. Someone can come an polish this up, but I believe that this may put some concerns to rest.