One year on, one of the disks has failed, and I realize that RAID1 for me is pointless.
Sounds to me this is exactly why you need RAID1- to prevent data loss in case of disk failure.
Did you run the manufacturers diagnostic tool on the disk? Did you try reseating the disk?
If you really want to downgrade to two volumes you could try taking all disks out, putting your new disk in and formatting it as individual disks, reinsert your original disk that was part of your RAID1 array. Not sure if your second disk will have any extra information on it to confuse the NAS though as it was part of a RAID.
Personally, I'd backup the data and start from scrath with two formatted, separate drives. At least you have a copy then should something go wrong.
fun_plug in it's simplist form is just a single shell script file executed when the NAS starts. It's not supported by Dlink. Not really sure what you mean about multiple ones. If you mean FFP and something else, that depends on your Linux skills. You will need to ask those questions at the other forums.