In the game, it's noted that Wylfred is taking his anger out on the Valkyrie (who here is Lenneth) for "taking his father away" rather than the person who killed him. To most people, this would seem illogical, but let's recall some details. Most Midgardians know that there's the goddess Valkyrie who take people as their Einherjar, but they do not know that there's three of them and that they're constantly cycled around each other. Given that they don't know this, what if before Lenneth was awake here, it was Hrist's turn as Valkyrie? You know, the one whose modus operandi for Einherjar-gathering is slaying worthy mortals personally? Then Wylfred's reasoning for "the Valkyrie took my father away" makes perfect sense!
One point in game is that it seems everyone you sacrifice to the Plume eventually becomes an Einherjar for Lenneth. Yet at the end of the game, it seems that Lenneth is aware of Wylfred's power and tells him to relinquish it. How does she know this? Well, aside from learning it from said people you sacrificed and the glowing Red and Black and Evil All Over Angel Slayer, take note of what song plays when Wylfred is contemplating on, and actually uses the plume on someone. It's the same song that plays when Lenneth senses the presence of the undead and dark beings, and you ARE using a Plume "consecrated" by the Goddess of Nifleheim, Hel. In other words, the song is a cue that not only means that you're going to sacrifice someone, but it's the same as telling Lenneth "this man has dark powers".
Wylfred's accusation of Lenneth taking his father away from him sounds more like something Hrist would do than Lenneth. Like most mortals, Wylfred doesn't know that Hrist and Lenneth are different beings.
Ancel's appearance and temporary joining of your party during the Ending A fight against Garm makes sense when you remember four details.
The first is that Lenneth more than likely heard Garm's appearance through her Spiritual Concentration ability and went to investigate it, as detailed in the point above. The same music that plays when Ailyth reveals her true nature is the same music Lenneth hears when sensing the presence of the undead and dark beings.
The second is that Lenneth allowing Ancel to join your party is the same as how in Valkyrie Profile 1, she allowed her Einherjar to retrieve relics or enact quests if you returned to places important to them. She's just doing that for Ancel, only this time it's joining in the fight against Garm.
The third detail has to do with why Lenneth hasn't joined your party. This is because while Ailyth was responsible for setting up the War of Succession, Wylfred did play a part as Hel's pawn in the whole debacle and signed the Covenant against her. If Lenneth intervened, she could likely kill Garm with her team of Einherjar, but Wylfred would still have points against him that could send him to Nifleheim when he dies. By sending only Ancel and letting him and Wylfred's party handle it, she pretty much allows Wylfred earn his atoner status and hopefully a way out of being sent to Nifleheim.
The fourth detail has to do with the God's notions of einherjar material. In their eyes, certain Einjerhar can be rejected and sent back to the cycle of reincarnation. Going by Ending D and C, Wylfred would be viewed as impure for his usage of the plume. But if Lenneth allowed Wylfred to slay Garm, recant the Covenant, make up with Ancel and commit to being The Atoner, it would mean she could be trying to open up the possibility of him being recruited as an Einherjar and sent to Valhalla, where he would be reunited with Thyodor and Ancel in the end.
In the game, the deaths of both Thyodor and Elsie causes Margot to lose her mind, and hallucinate Wylfred as her husband. The few times he's come home, she asks him to 'give her children' to keep her company. Could be another reason that Wyl is hardly home; he doesn't want his own mother to jump his bones
, and you ARE using a Plume "consecrated" by the Goddess of Nifleheim, Hel. In other words, the song is a cue that not only means that you're going to sacrifice someone, but it's the same as telling Lenneth "this man has dark powers".