She was desperate for a way to stop the Infernals and unwilling to take the risk of sending so may of her people out to almost certain death. She ended up taking the "Push the man into the werewolf's path, it is better that one should die than five." approach by declaring war on the Infernals to protect the Amurites and summoning the Mercurians to protect her own Elohim.
While one would normally think that the more militant Bannor would be a better candidate for summoning Basium, I'm not really sure that is true. In The Radiant Guard, Capria tells Falamar: "But beware for faithful Sabathiel told of stories of Basium, and he is unbound by mortal virtues." That seems to indicate that they see Basium as a lawless angel who does not fit into their orderly worldview. Junil probably sees refusing to follow The Compact as a lawless act, and thus Basium could be considered a criminal. Sabathiel knew Basium personally, as Ethne did not, so he would better know of the negative consequences that would come rom his presence here.
Perhaps the biggest reason that the Elohim built the Gate was because they were the only one who knew how. The Order of the Elohim has been around far longer than any civilization in the game, in fact it was ancient by the time Kylorin was born. While they were usually focused on philanthropy, they were also among the greatest lore masters in the world. Kylorin's life among the Amurites (along with the tendency of may in Elohim society to try to sanitize some dark parts of history) may have moved them up to #1 in knowledge of the Age of Magic, but the Elohim archives still hold secrets forgotten even before the Compact. They were the only mortals present at the signing of The Compact, and they may have been there when they saw Basium leave. Sucellus tasked them with taking care of his holy cites before he left, which left them with exclusive access to even more lore. I'm not sure when exactly the Elohim came across the secret to building the Gate, but it was a secret kept for countless generations. Ethne and Einion were quite frankly probably the only people on Erebus who had the know how, and as Einion would never build it it was Ethne or no one. Rathaus was sent in the hopes of killing Ethne before the gate was built, so the knowledge would be lost and the would would be safe from a war that would surely reach the Sidar eventually.
Of course, the Mercurians themselves also have knowledge of how to build their Gates, and they spread it rapidly to all their allies once they arrived. Once Basium stepped through that gate there was no going back, as he was created to embody the sphere of Persistence (as I believe is the meaning of Life, in the context of the 21 precepts in FfH).
