If they wanted to do it the right way they could've approached the new Highlord (or one their more-canon champions) with their request. Through Darion, Bolvar could point out that Tirion was prepared to become the Lich King and that this selflessness makes him a viable candidate for a horseman. They probably still wouldn't understand, but who knows, maybe they'd be willing to call his spirit back to propose the offer. They've called on Uther's spirit like four times now.
Instead the Ebon Blade assumes the worst and just slaughters their way through the chapel. Of course they're opposed.