I saved up over 900 idols. I bought a talent and noticed that the balance in parenthesis changed but the total did not. Does this mean I can still unlock objectives and campaigns because it goes off of my all-time earned idols even if I used them?

Yes, idols spent on talents still give you DPS and gold bonuses, and they count towards unlocking objectives as well. Basically, the only reason to hoard idols is if you are saving for an expensive talent. Otherwise, spend them, always.