Question

2 situations both at comp rel.

1) my scarab god dies on my opponents turn. He passes turn. I untap and say upkeep. I notice my scarab god and announce for the first time. Scarab god was supposed to return at end step.

I missed the trigger and do not get it back.

2) my scrab god dies on my opponents turn. I acknowledge the trigger as it dies saying it will return at end step. Opponent passes turn. I untap, upkeep then mention my scarab god was supposed to return.

I ackowledged trigger but failed to resolve at the appropriate time. I get my scarab god back.

I believe I have the correct interpretation of each situation.

Neither of these are rules questions, so they don't really belong here; I'll move this thread along to the Competitive REL forum for you and open it for discussion.

Scarab God trigger is delayed trigger

A delayed triggered ability that changes the zone of one or more objects defined when the ability was created. For this trigger, the opponent chooses whether to resolve the ability the next time a player would get priority or when a player would get priority at the start of the next phase.

So the God will be back anyway

To expand a bit on Evgeniy's answer, the first trigger “When the Scarab God dies” does nothing else than creating a delayed trigger, so it resolves automatically, regardless of whether you mention it or not.
It is possible to miss the second trigger, but then the fix is to resolve it when it is noticed.

Evgeniy and Florian covered the policy regarding this type of trigger - Obzedat was the “poster boy” for a long time, now it's this Scarab dude … but the policy remains solid, regardless of the currently played card.

d;^D