Been googling for some answers.... but recently encountered a delay whereby the app release is still pending review, thus afffecting my planned release time at 8am.
1. You can't - there is no way to secure a pre-set release TOD, only that the app is auto-released to the store once approved (requires the dev to pre-set that configuration).
2. Depends on the depth of the queue and how long it takes to re-index on the backend, etc.. Matter of hours, usually.
In the future, best practice would be to get the app in the store first, temporarily taking it off sale in all countries as you move ahead with other plans.
Also, don't count on the app being approved at all, or that it will only require one review.