Ok, let's see if I can help ^^
Yeah, you're mostly correct, even though there's a fine combination of things that have to happen at the same time for it to be ^^
After you've determined the amount of attack dice rolled by the detachment, it does not matter if it's coming from any specific unit of it, so yes, you can use scouts to extend the attack range of a detachment as it will allow you to allocate more hits on extra enemy units, at the cost of it also allowing enemy detachments to assault them and rout your whole detachment due to overwhelming melee superiority.
That said, the gretchin
must be part of the same detachment as the orks, or you'll need to split fire.
It is something we've actually been discussing a bit on the Epic Remastered
FB group, and there's some mixed opinions on it, but other than that, your interpretation is correct, and a single Epic
40k phase usually encompasses a fair amount of time and maneuvering.
For rules, there's two main digital venues you can check to have all the info:
- The Epic
40k Compendium (
https://thehobby.zone/resources/e40k-compendium/Content/Home.htm) with all the original rules, errata and additional rules from all publications
- The Epic Remastered Project webpage (
https://epicremastered.com/core/index.html), where you can find a rewriting of the rules for ease of reference and clarity, plus some QoL improvements and organic developments on it