What evidence points to Andruil, Ghil, and June being on the anti-Elgar'nan alliance? 
The evidence is circumstantial (as always) but builds up quite a bit in the larger picture. It's a combination of these three being heavily hinted at as being interconnected on a functional and personal level, added to the evidence of direct threat to Order described in Andruil's TOM codex and inferred prior bad behavior and punishment from her mural, topped off by Elgar'nans frustrating inability to give them his standard say-hello-to-the-Void "Forgotten Ones" treatment thanks to his reliance on Ghil and June as tools to maintain Order. (Not just in terms of needing OGS worship vehicles or weapons of war- they actually make the prison-forms that allow him to effectively neutralize all other threats.)
First, Andruil's connection to Chaos (the natural counterpart to Elgar'nan as Order) doesn't require us to accept a core OGS-temperament connection with Sera, though it certainly doesn't hurt. Her own history speaks to that subject if you read between the lines. In her temple mural, Andruil shares a suggestive circular pattern of forehead tiles with only one other member of the Pantheon, June: a connection heavily hinted at being representative of an ancient Arlathan tranquility brand. We're not given the details of her transgression, but we can infer it was serious enough that 1) Elgar'nan felt removing her access to magic was justified, and 2) Mythal did not intercede on her behalf. In this light, Andruil's trips to the Void to go "hunting" take on a very different light: she is hunting for allies, not prey. Her intimate connection with Ghil protects her from being tossed in herself, but she can't fight Order alone, and her ability to even be a thorn in Elgar'nan's side is critically hobbled at this point. (It's also easy to imagine a number of prisoners there being her secret, Red Jenny-esque compatriots- ones that did not share her immunity to Void punishment.)
Like Solas, Ghil's contribution to the cause (the pre-rebellion Cold War of Chaos vs Order in ancient Arlathan) lies not so much in taking direct action, as in what she prevents Order from being able to do. Her personal relationship with Andruil and the Pantheon's reliance on her creations forms a limited but critically protective shield allowing Andruil to act. Similiarly, Solas also prevents threats to Order from being easily or permanently neutralized, as Falon'Din's OGS domain prevents the permanent death of his allies, making their imprisonment in inaccessible monstrous forms within the Void the only viable punitive alternative. Again, forcing Elgar'nan and the alliance of Order to rely on Ghilan'nain and June instead.
And for a while, this seems like Elgar'nan's best option. Falon'Din (and other Forgotten Ones that may have joined his failed rebellion) represent a far more serious and organized threat to society's Order compared to the relatively mild irritation of Andruil's random acts of subversion. However, this changes drastically with the emergence of red lyrium- a bright and shiny new WMD available to both sides (it was Ghil's "oops" that created it) that immediately threatens to turn the Cold War hot. June's loyalties, already hinted at fairly strongly by his collaborative connection with Ghilan'nain in various creations including Void prison forms, becomes quite clear in the formation of Andruil's armor and weapons. This is not something she or Ghil would have had the craft to do- it demands June's direct and willing involvement.
At this point Mythal sees the threat for what it is, and strikes a deal with Falon'Din/Dirthamen: she'll release him from the Void and restore him to the Pantheon in exchange for wiping knowledge of how to access the Void from Andruil's mind. (And by inference, likely that of Ghil and June as well). He agrees, adding yet another "betrayal" to his ever-growing list, this time on the side of Chaos and the Forgotten Ones. Solas is released as Fen'Harel- critically into Mythal's direct custody, where he immediately begins to plant seeds of doubt in the future of the Pantheon as it stands, and coax her instincts regarding the protection of the People into play.
When Elgar'nan continues to experiment with red lyrium as a tool for enforcing Order (evidenced by the crafting of his new focus, as well as the petroglyph-like paintings of red lyrium slaves under the symbol of the sun painted by survivors of the ensuing civil war) Mythal can no longer ignore what he has become. At this point, she, Solas, Andruil, Ghil, and June (perhaps others, but that's still uncertain) join forces in full rebellion against Elgar'nan's Order and the great Civil War of Elvhenan begins.
*rubs her temples*
I hate trying to shove tenuous brain-theory into words. Never comes out right. Let me know if you buy in and want more on the Ghil / red lyrium connection- will dig into theoretical details of how and why.