Dr. Just to clear the air, you are more than welcome here. In other words what I meant to say was slow down a bit, revisit the fundamentals of Epicor Under The Hood and you will struggle less. You are rebuilding Job Entry to make it easier for your users, but it will be a long road unless you give yourself 14-60 days to get the fundamentals down. As soon as you asked about GetList, GetDataSetForTree and Tracing 101, it was clear that you needed to slow down a bit. Confidence can be a double edge sword.
You do realize every upgrade you will have to do rigorous testing, Job Entry isn’t ABC Code Entry. 1 Mistake and MRP Crashes, 1 Mistake and Scheduling Fails, 1 Mistake and everyone in the company will come to you and say “Yechezkal! YOUR SYSTEM DOES NOT WORK!” As if you wrote the whole ERP Application, suddently Epicor becomes “Your System”.
We try not to spoon-feed as a community, often times we do - but we’d rather guide towards the answers and teach someone to fish. Sometimes those answers may sound like a “fu” to a person, but the intend always is, you will thank me later. Just ask @Hari_Dutt who just wants a simple BPM 
since February.
We want you to keep posting, but give yourself some time to read the Architecture Guide, ICE Tools Guide and understand Epicors N-Tier Application Setup, how it utilizes Entity Framework and how it has “Entity Framework Triggers” (not db triggers). BELIEVE Me you will stumble upon a simple method that will keep you there stuck for a month all because there is internal Trigger Code overwriting you. You will start dreaming about a villain called “Row has been modified by another user” chasing you and trying to eat you.
I understood your intent as you want to learn the Framework and Epicor, and become the best one can be, so I guided you towards that – slow down, hit the books and practice, practice, practice.
We also try to guide folks towards the right solution, for example you want to make the Job Entry easier because your Mfg Folks struggle with it? To me that is not a reason you should customize, it just means a failed Training Program. We have a VERY Complicate Job Entry - a single Job can have 1000 operations and 50,000 materials, 80 Sub-Assemblies, 100 Child Jobs, building Vehicles 1 mistake and guess what happens? RECALL, Government Fines, Millions of Dollars Lost and as a Publicly Traded Company we are under Investigation and our Stock Drops. But somehow our English and Non-English speaking Computer illeterate people navigate it just fine. Perhaps Re-Train and hire someone who specializes in Training who speaks to the people, who can mix emotions with logic and resonate. Plus Job Entry should always be accessed by Engineers and Subject Matter Experts and if they are struggling, do you have the right people for the role?
I Customized the Packout HandHeld Screen for 3 months, a tiny little itsy screen spitting out labels, how hard can it be? Every upgrade I find myself debugging as Epicor adds more PCID stuff to it, breaking my stuff. I remember @Chris_Conn staying awake for days to Freight and Unfreight a Shipment, how hard can it be – we got the trace… I guess its much harder in practice. @josecgomez was struggling with Updating a Job due to an Epicor Bug he had to use UpdateExt, everything seemed simple, this was after he already had reached his 6+yrs of Epicor Under The Hood Experience.
Try understanding Epicors “Phantom, Non-Stock, Purchased, Manufactured, View As Assembly, Pull As Assembly” flags, simple checkboxes that make or break your mfg process. Before we even began to Customize that portion, we ran 60 tests to understand the cause and effect. Not a single Epicor Guide or Epicor Specialist could help us, it was hands-on and practice, practice, practice. I doubt even Epicor knows about it as much as we do now, definitely all the consultants we had were a joke and the only contribution they gave within a 8hr/2 day period was… “Did you know, if you right click the tree, you can resequence?”… “No Sh*t, where were you the other 7hrs to contribute something, while we pay for all your travel expenses” lol Paid 5K for a Resequence Tip.
Many other Screens are like Job Entry, are you going to rebuild them all? Perhaps your company has chosen the wrong ERP System to begin with, or its a proper staffing issue. We upgraded to Epicor from IBM AS/400 (Terminal) so you can imagine the challenges we faced going from DOS to UI… “why doesnt the enter key, tab anymore?” - “Because use the TAB Key to Tab”. Also IBM AS400 is FAST!!! I mean FAST!!! MRP Full Regen 45min… Epicor 8hrs.
We just upgraded from 10.1 to 10.2 and immediately a request came in to customize Logged Invoice Entry, because Epicor now makes you search Purchase Points and not just Suppliers… But on the UI You can Filter by Supplier, the team immediately said “WE NEED A QUICK SEARCH!!!” The response was “No you dont, you need to train them how to filter it, because you will see this exact same issue on 50 other screens, are we going to customize them all?”… Sometimes Better Training, Training and Re-Training is the solution. EKM (Epicor Knowledge Mentor) can assist with making EUPs and Videos.
You have ambition I give you that, but always ask yourself am I solving the cause or symptom. Example: With the CORS Issue you were happy to have resolved the symptom, but not addressing the cause of potentially corrupt Instance, before you know it - you will hot-patch 30 other things. Hope all works out! Good Luck and Welcome to the Forums.