So with the latest Famitsu article we can pretty much conclude the list of crests our main characters have. We also know a fair bit about their (crests') connection to history. The confirmed crests are:
Saint group: Crest of Seiros, Crest of Cichol, Crest of , Crest of Indech and Crest of Cethlenn. We also have a mention of Divine Macuil, the crest of whom is probably the one on the right of the Crest of Chichol if we are looking ate the mural. The main reason why i think that is her(?) crest is because statues shown in the article have weapons with a fair resemblance to their crest image (Chichol is actually is the most obvious one) and Macuil is holding a sword which hilt might actually be an inspiration for the crest design (also all saint crests so far have been based at the top of the mural).
10 Elites group: Crest of Dominique, Flardarius, Daphnyll, Blaidydd, Gloucester, Goneril, Gautier, Charon, Riegan and Lamine.
And that is 15 crest confirmed. But we have 21 on the mural excluding the crest of Flames Byleth has (since it is in the middle and covered).
That leaves us with 5 (the crest of ?Hraesveleg? is not confirmed but we know of revealed bearer and relic weapon) crests of unknown allegiance. All of them are either damaged (Marianne's crest) or damaged on the basic mural. Below is the mural with red saint crests yellow/blue elite crests and purple other crests.
Which kinda led me to a little speculative theory which has several main points:
1) So we know crests originated from a certain high being. This certain high being is not depicted ANYWHERE and nobody worships it, even tho crests are deemed as very important and powerful. this is very much weird. That leads to the second point.
2) Either saints (which i see as more likely) led by Seiros or somebody else had a falling out with the high being which led to a wide conflict. My ideas include: imbalance of power (this idea is based upon the fact that Byleth's crest is placed in the middle of a mural, which indicates it is the most important . So maybe Nemesis, the previous know bearer? of Crest of Flames held too much power so divines got jealous or smth.), disagreement between policies (maybe saints wanted to worship high being and SET UP A CHURCH, but as the main representative Nemesis refused) or corruption (in this one everything gos well until Nemesis gets smhow corrupted by his crest and starts rampaging so Saints have to stop him).
3)Since Nemesis lost the war all infromation about high being was restricted and people maybe forgot and started instead worshiping Seiros and 4 of her companions as saints. That may explain the 1-st point i made.
4)I think that maybe 6 people initially joined Nemesis in his "Liberation" (thus 6 damaged crests on the mural), maybe 10 greats remained neutral or joined Divines. Main thoughts here are:
Bearer of ?Hraesveleg? crest may have betrayed Nemesis, but to keep him in check/reinforce friendship/maybe he fell in love, he(who knows) had to marry Seiros and thus Adrestian Empire was formed. Thats why this crest is not completely obscured on the mural but it is not really mentioned (smth like shame bearers of this crests hold).
Night crawlers are either companions of Nemesis or their descendants who want to ressurect the Liberation King (who knows the might actually succeed with the hands of ?Nemesis? bursting out of the coffin in one of the videos). That might explain their hate for Church of Seiros and mysterious powers (they might also be corrupted crest bearers).
Thats basically it or the theory so tell me what you think plz.
P.S I'm bad at image stuff so this mural edit is made in paint and is BAD.
P.S.2. English is not my first but i tried my best.