RN104 raid failed

Starting a re-sync with a replacement drive and another with that many errors without first making a data backup was a big mistake.  Unfortunately, it may be one that resulted in data loss because your volume is not redundant during the re-sync and a failure of any other drive can (an apparently did) damage or even destroy your volume during that process.  You should also use the full admin GUI, not just RAIDar in making any decisions.

Whatever you do, do not attempt to put the original Drive 1 back in.  If it still works at all, it could be of some help in data recovery.

The NAS is showing data and data-0 because it sees some of your drives as a partial volume and some as another, both with the same name, which is not allowed.  data-0 is the name of the MDADM RAID on which the BTRFS volume resides, so it's using that for one name.  That's probably the new drive and won't show up if you boot without that one  So, the first thing to try is booting the NAS with just the original 3 drives in read-only mode (controlled via the boot menu).  If that works, make a backup right away

Unfortunately, if that fails, it is rarely successful to try and mount the array manually when the automatic mount fails, so you are likely looking at a data recovery situation.  Netgear does offer data recovery, but it's expensive.  Software such as ReclaiMe and Home NAS Recovery may help if you have a way to connecting the drives to a PC, but they are also not cheap.  Both will allow you to assess your chances of recovery before you shell out the cash.  ReclaiMe has been used successfully by other forum users.  Home NAS Recovery is a newer player, and I have not seen anyone come here who has tried it.  But it claims to do a better job with a damaged volume.