All cells can drain similarly, and your IR can be high enough on each cell and cause a meltdown. You wouldn't notice that without a meter. It's not typical, but possible. And the IR will climb on all batteries over time with use and wear. Keeping an eye on your IR is the best way to know the...
They all test voltage, but not all test IR.
The IR will vary, depending on a lot of factors. It's best to get in the habit of monitoring your IR the same way every time to get a comparative value. I let my battery charge, then sit for about 5 minutes. That's when I read my IR.
Yeah, each cell is charged individually. And it will test the IR by putting a load on the individual cells.
In a perfect world the charger will charge all cells evenly. But since each cell will have slightly different IR's, capacities, etc, when one cell gets topped off it continually drains...
As long as your batteries aren't sitting near anything flammible, a bag isn't going to do you much good. The smell is worse than the flames in that case, and a LiPo bag isn't going to prevent that.
It's your average run of the mill charger. It has a 1A balance current as opposed to the Hota D6 Pro which has a 1.6A balance current. It means your batteries will take a lot longer to charge if you're doing the bigger packs. 80w chargers are pretty weak.