Herald are really not that impressive.
Of course not, they're a free unit thats only use is
to deliver messages.but in noway unique or even special besides being a favorite of the god
they would still have to avoid death if they tried to give a message to a enemy
Er no. Depending on how the god wants to RP, the herald is effectively mortal and can appear and disappear at their masters will (if their master is not a demi-god.) However they are not adept at fighting against a god (or exarch or god-ish)and will fail, they can and fight pretty well against mortals, so they are not over powered. So basically, a Herald is an immortal RP tool.
Herald->exarch->God-ish->God
Er no.
It should go herald -> God-ish -> Exarch -> God.
A god-ish has no CR and thus effectively fight well against gods, it has no use in a fight other than as a RP tool. An exarch has basically 1CR (abstracted from their 'counts as a battle artifact') and can fight against a god (albeit not well) and in most cases defeat a god-ish creature. A god-ish creature is basically a really powerful mortal, a really powerful exarch is a really powerful mortal, given gifts from their gods (varies from god-to-god, exarch-to-exarch.)
under the rules here they are technically demi-gods.
No. Really, no.
They're a hero god, I'd also bet the god-ish creatures are a little weaker since they don't gain AP, and do not have a primary domain.