Right. I forgot to consider the possibility that the "Anti-Monster Field" would get stronger the deeper we go into the temple. Naturally, it is tradition that the Angel will be chained right next to the emitter, deep inside the bowels of the earth, guarded by deadly traps and deadlier guards. Seeing this, I agre that we must leave the Grand Temple alone by the time being. We simply lack enough party members to do everything else and this quest at once with a reasonable chance of success: we would need Zayan for SCIENCE! support, Rin for combat/distraction, the twins to cover weak spots and Lucy to oversee the operation. However, even then this isn't enough: the twins might be affected by the "A.M. Field" if they happen to have inhuman ancestry (which in Aria's case is likely), turning them into a liability. Moreover, if we encounter The Priest at any point, we can almost consider the operation a failure already.
That's why I think we should switch going to Yuiria's Grand Temple for Ontoglia (talking with the Adventurers' Guild is fine though) for now. Once Zay has enough magnetite, she will able to assist us with extra firepower and gear, such as hazard suits to shield our party and maybe a magnetite-powered drilling vehicle to bypass the greater part of the enemy's defenses.
Additionally, even if we were to succeed, nothing says the Angel will be cooperative to begin with: considering their reputation and possible origin as weapons of war, he/she/zir might actually don't mind to be a slave of Methuss, as long as he's promised the chance to go wild and unleash carnage unto whoever happens to be the current enemy of the kingdom. Alternatively, he may not like being imprisoned at all and won't be too willing to talk with us until he's released, moment in which he'll immediately proceed to butcher everyone in his path (including Erdrick and co). Therefore, Ontoglia is the safest choice due its lack of unknown factors and possible dangers. And sending any goblin to the temple is a certain death for the critter.
Nevill, I understand your concerns, but it is unlikely Methuss would unleash their ultimate weapon in order to search for the missing ore. Same case for the adventurers' expedition: even if there's a reaction from the capital, unless the Kingdom felt there's something powerful enough to be considered an extremely dangerous threat, common sense says the angel will stay wherever it is until they believe the situation has changed. Granted, after we deal with the incoming incursion within our territory chances are the status quo will change, but by then we'll hopefully be prepared for such eventuality and be stronger/have more resources and manpower.