i don't think some people understand what dumping and flashing via hardware means. it's not a 'trick' that nintendo can patch out, we have hardware access. it will work
That depends, actually. If there are other devices on the system which have their own firmware that get upgraded as part of the upgrade, restoring NAND back doesn't mean that those devices get downgraded as well. If their firmware has any reliance on new features in the NAND software... well... paperweight time.
Now, does that mean it happens here? Doubtful. AFAIK this is more of an issue with devices like cell phones where you have several different firmwares that are usually bundled as part of a device software update.