Simple, supply versus demand. Low supply for second gen and demand for it causes prices to go up. Don’t overthink it. We’ve seen this trend before in the past with other parts, but definitely not to this extent.
Likely the rear camera is just bad or damaged, when it is the LED flash will not work. Also your loudspeaker replacement probably should have just been a grill mesh clean/replace. Unless water actually got into the speaker then it would explain why the rear camera failed.
If multiple batteries are still draining then you'll need to check you Tristar (aka Power IC). Do this by leaving the battery unplugged and connecting a charger to the phone, a working phone will show an Apple logo and boot loop. A phone with a bad Tristar will not do anything when you do this.
It is after market screens, we see this every year with the new iPhones. There is always something manufacturers need to work the kinks out. The 5C had bad touch issues, the 6 had frame lifting issues, 6s with force touch and now with the 7 it's the aftermarket force touch/backlight again. We should be getting answers from suppliers. And iFixit screens do the same thing, I've tried. I've only had a couple screens from different suppliers work. It's like a needle in a haystack right now.