ECC support depends on the processors that the NAS uses. A few of their NASes allow you to use ECC memory but you'd need to swap the memory installed to ECC memory. A lot of their systems use Intel cpus that don't support ECC at all so you need to carefully pick and choose.

Don't buy a QNAP if you care about real ZFS. https://news.ycombinator.com/item?id=48593360

Some do. I got the TS-873A a few years back, it works. Their software is kind of weird, and I wouldn't connect it to their cloud offering, but it does work.