Is this really still possible? Rhitta is supposed to absorb the heat that comes off the user's body during the day, but Dawn directly transforms, converts, and then stores the heat from the sun. So there would be no heat emanating from Gawain's body to be absorbed by Rhitta (unless Gawain charges it herself).
Rhitta was made especially for Escanor and Sunshine. In Gawain's hands it's just a quality weapon without any special power. Otherwise i think she would have summoned it against TB and used Charge and Fire.
I still believe Gawain will have her own ST.
The volume extras
here mentioned that the sword has the ability to store the power of the sun. I suppose that could be interpreted as it storing the heat of the actual sun, but given that the weapon is literally just a fragment of Escanor's axe and shares the same ability to fly to the user I think it would make sense for the energy storage property to be the same one as well. You are correct that Rhitta was designed for Escanor in mind and that it isn't the ideal weapon for Gawain, which is all the more reason for her to get an upgrade. With the reveal that Beltreipe is Daymond and that he seems to be unconsciously reacting to Rhitta, I think it's possible that he could end up destroying the sword during his final fight with Gawain. This way the end of that duel would simultaneously involve Daymond finally getting over his brother and Gawain finally stepping out of her predecessor's shadow. It has a nice ring to it.
I reread the vampire side chapters and the vampire king didn't do what you mentioned, all he did was try to cover Escanor (only) with "impurity" (which seems to be a kind of dark attack that destroys the body, as we see it destroys the body of a mouse as soon as it touches it)
Well you might want to give it another go because the vampire king mentions his darkness envelope in response to Escanor's comment about morning:
The same thing is also alluded to by Merlin during the meeting with the holy knights earlier when she mentions that Edinburgh is covered in darkness, and we can later see the envelope coming undone when the castle suddenly becomes very bright after Escanor defeats his foe. The point of this technique is to protect the vampires from sunlight since they are vulnerable to it. It's different from the attack you're talking about.
Your question is similar to asking why the Sinner didn't use "full counter" on Mael's attack and your answer has two possibilities, either the Sinner can't use Chandler's abilities even though the latter is half of him or the sinner is an idiot
I think Mael will not be able to use Sunshine too if he inside "true night"
The Original Demon should have been able to use the Darkness spell because he identified it as belonging to him when it was broken. This is notable considering that when Chandler and Cusack woke up after their defeat they mentioned that they were mostly unconscious while fused together. Cusack's consciousness did briefly resurface at one point just before the Original Demon charged Mael and got nuked, which could perhaps explain why the demon acted so foolishly there. He wasn't in his right mind at the time.
I think it would make sense for Mael to be able to dispel the artificial night even from the inside given that he was willing to get close to the Original Demon despite the risk of the latter reactivating the Darkness spell. If all else fails Mael could always just do the same thing Escanor did during the Great Fight Festival and briefly activate Sunshine via a mental image instead of the real sun.
Percival can't possibly lose to Diodra. Balin VS Diodra would be extremely one sided. Even if Despair can reflect damage, it'll be useless since Invincible cancels attacks. Diodra's fighting skills are bad, he wouldn't be able to do anything.
I'm not so sure that Invincible would cancel the effect of Diodra's magic since Despair isn't really an attack. In terms of magic types you could think of the damage reflection as shifting-type magic that swaps Diodra's condition with that of his attacker. If Diodra is injured then the same injury manifests on the attacker's body and if Diodra is poisoned then the effect of the poison manifests in the attacker's body. Invincible keeps Balin from taking damage, but it's not clear if it could block a status swap like that. Compare to how Invasion managed to bypass The Ruler due to the former not being classified as either an attack or healing.
No she won't and Gawain beat Tristan too
My money is on the tournament being interrupted during that fight. With Gawain having burnt through some of her reserves it's plausible that Tristan would have the upper hand in that encounter, until Isolde goes to confront the second Isolde and gets her to turn off the memory magic just as Tristan is about to hurt Gawain. When Tristan regains his memories he'll become hostile towards Beltreipe, thus breaking the effect of the magic ring and causing Beltreipe to realize what's going on. Three KotA vs one Perils would probably be too lopsided so I expect Ironside to show up as a spectator so that Percival can fight him for a bit. I guess Pellegarde could also be present to give Gowther something to do. They last met only a few days ago, which means Gowther would be able to recognize that something weird has happened and connect it to the Chaos influence that he has already observed inside Beltreipe's mind.
I know some people want a Percival vs Tristan fight because of how Tristan reacted to Percival's death, but such a clash would be pointless unless Tristan has his memories back, in which case he would prioritize defeating Beltreipe and escaping Annwfyn over taking his anger out on Percy. If they're going to fight it should happen later, perhaps after Tristan has obtained his ST since he needs a buff to keep up with the other KotA.