the outer barrel should sit as far back as it can go, but it might want to sit slightly forward of that position to get the hop bolted down, although in your case you seem to be getting the opposite problem.
generally i've found that they can be tight to get together, but that same tension keeps them solid once it's assembled. a qc issue is certainly plausible as with everything airsoft.
the gap between the top cover and the receiver is pretty normal, i wouldn't worry about that as long as it closes properly.
it's sadly one of those fitment issues that's all but impossible to really diagnose over the internet, you may well be right that you've got an unlucky qc fail in the receiver.
for the flash hider? i'd assume it's a case of the typical 14mm ccw thread with the adaptor to fit 74 pattern cw threads, e&l are noteworthy for actually casting the proper threads direct to the front sight block.
The 5KU B11 is very loose at the end which meets the receiver on the Cyma if memory serves. I struggled to find any Zenitco (be it real or any of a number of repros) that fit the Cyma well and ultimately did the opposite of you and bought an E&L. Granted at this point I'd given up on the idea of doing it myself and sent it off to have a real B11u machined to sit properly in the E&L.
TLDR, don't bother fucking with AKs. There is nothing common spec wise across them so nothing ever fits properly without bodging.
I'm disappointed that I've had so many issues with the gun.
It literally hasn't left my room since July 19th 2021, when I had the mosfet overheating issue (caused by a dying motor) on its 4th overall game. Since then it all went downhill, the motor finally died, then I bought a warhead, warhead died, waited one month for the repair, then the gearbox casing fucked itself, bought a RA shell, now I found out about the receiver..
I like the looks, the wooden parts are fantastic, but I'd take a working gun over a beautiful one every day of the week, and in its current form, this gun is far from being in working order.
hmm, i'll admit there's only 1 aftermarket box i ever dealt with in an e&l shell and that was a fusion engine. from memory it needed wedged forward in the reciever although not by much.
Doesn't move further.
View attachment 92635
I guess this is the correct placement, if there is a way to make it go further back, let me know.