There's a lot of places the file can get corrupted on its way to your drive. The memory of the NAS itself is only one of those. If you want any certainty you need to verify it after writing, so ECC RAM is not enough. And once you do set up that verification, you don't need the ECC RAM anymore.
I am not sure this is correct - there could be an error not only in the data, but also in instructions - but flip could cause data to be written to an incorrect location of the hard disk.
Possible, but I bet the risk of writing/losing the wrong sector based on that kind of RAM bit flip is a lot less than the other sources of writing/losing the wrong sector.
That differs based on the program you're using to put files onto the NAS.
But I'll note the even with ECC you need to double check things in case there was corruption on the drive wires or in many other places. With the right filesystem you can find some of those locally during a scrub, but double checking end-to-end isn't much harder.
