RN102 and Active Directory domain

The AD auth works (at least with server 2012 R1), setup is simple, all users and groups are shown and you can set rights on shares as if they were local users. local users are disabled if AD is enabled. Netgear chose to display rights by share on the interface, so you'll have to set the rights from a windows computer going into the folder properties if you want per folder rights and not per share.
I believe the OS6 is the same for all the Netgear device, pro or not, so every function is available for everyone. You got an interface to backup shares. Rsync is a sharing method which is supported, you can use it over SSH too. If you want per folder backup, you'll have to drop into SSH to do it though.
Netgear has a built in NAS replicate feature, but I don't know if this is available for old devices or only between OS6 devices.

I also have to say that some users are experiencing problems with OS6 stability causing transfer crash and device lock up (solved only by waiting for hours or doing a power cycle) when you do heavy write on the device, an update is to be released soon to fix this. I advise to wait for 6.1.5 or 6.2 update before buying. It seems to be a pure software problem related to BTRFS, so I assume this should be fully solved with the update, but until it's fixed I cannot recommend the device for professional use. But since there are rumors of the update coming in by mid-december...

For the choice of the NAS, the RN102/RN104 may be limited in speed. I personally get 100MBytes/s in read speed and 40MBytes/s in write.
My setup SMB over gigabit network, RN104 with 4*4Tb seagate NAS HDD and the client is a computer with I7-2600K, 16Gb RAM and 2*WD7502AAEX raid 0 under windows server 2012.
Those speeds are enough for a small environnement but I advise to get a more powerful device (RN3xx/RN5xx ?) if you have more than 5 simultaneous people on it.