My AEA Backpacker arrived today. It came with the same stock yours did. I'm probably going to eventually replace it with something nicer, but at the moment AR parts seem to be quite pricey! I also saw where you said the stock connection was loose. I fixed mine, but not sure how long it will last. I had some 2mm thick sheets of high density foam that were packing from something I had ordered. I cut a square of it, then cut a hole in the middle big enough for the post to go through, then applied some weight to the butt, compressing the foam, as I slid the pins back into it. I then trimmed the excess with an exact knife so you couldn't see the edges of the foam. So far, so good.
It came with 150 Bar in the tank, I filled it to 250. The first magazine was messing up, it would fire but only chambered a pellet every other shot. So, I'd shoot, a pellet would hit the target, the next shot was just air, then it would shoot the pellet, then air again. This just happened on the very first magazine shot through the rifle. I have shot 4 more full mags through it and haven't had another misfire. I stopped because my finger was getting tired! Yes, that trigger definitely needs some work!
Is this the spring kit you have?
If so, do you recall which spring you used? Did you have to degas the rifle before you pulled the trigger assembly? I think I'll watch the video again on the trigger and give that a go this evening. Right now I'm just shooting the cheap Crosman Premier Domed pellets to break it in. So far it seems pretty accurate, giving me 1" groups at 40 yards, which I don't find too offensive given the 40 lbs trigger weight! Hopefully, a lighter spring, a good polishing and some lube will help lighten it a bit and make it a bit smoother.