First for the anonymous. In Windows this means that if you are using an account that doesn't have a password or that password isn't the same as what is used on the DNS-323 it will try to log in anonymously.
So if you go with the username/passwords for one share you have to do it for all given the current limitations of the DNS-323. As for the comment that you can't have a "public" folder, well that depends on what you call public. If your idea is anyone that jumps on your network with or without a password should have access to the folder then it won't work once you restrict one share. But if your intent is what I think you mean by public, as in all the people in your family should have access to that directory then what you do is make sure all those users are in a Group like Users, and then in the Network access you give the Users group access to that folder. So unless you have strange people browsing your network the difference between the two kinds of "public" don't matter.
On the "Mapping" of a drive that you didn't understand. Let me see if I can explain.
First off in the Windows world when you want to access a network drive/folder there are two forms to do it.
One is called UNC (Universal Naming Convention), it is in the form \\ And then the machine name \ And then the share name \ And maybe some more directories/folders under this. So \\myMachine\share describes a full path to folder that is shared by "myMachine". The machine has to share it, you can't just access anything you want on that machine. You will notice that the traditional Windows drive letters A:, B:, ... are not being used. When you browse using "Network" in the Windows Explorer it is using these UNC syntax. But what if you have a program that is expecting the X: kind of syntax (as in it doesn't really know about network drives)? Windows has what it calls "mapping". And what it does is allow you to map a path like \\myMachine\share to a drive letter like S:. So in a program you can use S:\ instead of \\myMachine\share. Each user can have different maps. The mapping can be performed by right clicking on Network in the Windows Explorer. Also one thing about this mapping. I should point out that no matter if you try to access a file/folder through \\myMachine\share or S:\ Windows will in the background log into myMachine so that it can be determined that you are allowed to access this folder/file. Once you have a way to access the folder(s) you want on the DNS-323 you can treat them just like any other drive on your machine, and so you can use Windows Explorer to create them if you want.
On the idea of sharing a directory/folder. In Windows you might right click on a folder and select the shard option, and in the process tell who can access it. You are doing the same thing on the DNS-323, but the process to set up a share is different on the DNS-323 and that is why you are provided with tools in Network Access. First you have tell it about users and groups, and then what they can access, which implies that those directories/folders will be shared out using those permissions.
On the Raid 1. If you have another way to backup then I guess you are good to go. BTW another thing that messes up data that mirroring doesn't help is viruses. To me though the hassle of Raid doesn't justify the very few times you really lose a hard drive. If you look around you will find that people have problems with it getting out of sync, they have problems even after a hard drive fails just trying to reconstruct it. It is far from a no brainier that anybody with no experience is going to be able to handle with no problems. As a point of reference I have maintained corporate software build systems with 30+ machines and they had to be up all the time, and of course they did have raid. But I don't run raid at home. I don't think it is worth it, and frankly the professional RAID systems I used at work were a lot more reliable and even easier to use, for things like rebuilding (reconstructing) a bad drive. On them I could even change out the bad drive with the power on.
I love the very fact that I have ALL my data stored on the DNS-323 and that I have it setup so every night it backups from one drive to the other, and to send it to offline backup, and I don't have to have any of the computers on to do it. But each to his own.