I agree that Aluminum could be Adonalsium's God metal as it generally either blocks or affects different forms of investiture in weird ways. As well as the fact when you make it into Duralumin on Scadrial it has the opposite affect and grants a huge boost to investiture. I think it would be very interesting to see how Duralumin would affect Surgebinding. As for the reason that it is possible to make Aluminum, while it is nearly impossible to make the other God metals without a shard actually makes sense. All the other God metals have shards that have direct or somewhat direct control over the creation of their God metal, because they have and active Intent and Command when it comes to their God metals. Adonalsium being shattered spread his God metal across the cosmere and outside of his control as he no longer has the conscious control over where his God metal is produced. Aluminum is also able to be forged relatively easily, while other God metals tend to be nearly indestructible is also due to Adonalisium's splintered state.