Jealous of the cabled house
OK, as you can connect the TC, NAS, iMac and AppleTV by ethernet you have an easy and robust config.
To your specific questions:
1. Although my wife and I both use the iMac/Macbook, we are set up under one user and are fine leaving it that way.
I meant users as in more than one person using the network at the same time etc. No problem either way.
2. I'm flexible in terms of how I connect either the TC or the 210j. At least 1, and possibly both, could/would be connected via Ethernet to the iMac.
I'd connect the Time Capsule to your broadband modem (I'm assuming there's a modem in the mix), connect the Synology NAS to the TC. Any device on the network now has access to 6gb of storage.
3. The other consideration is iTunes. I will likely leave music/video on the iMac given that we also use Apple TV. I believe Apple TV must reference files from the iMac/iTunes, rather than from the 210j. True?
Mainly true. To access the home sharing on AppleTV to play music/video from your iTunes library your AppleTV needs to be able to access iTunes on your Mac across the network (the the library can be 'anywhere'). Now that you can keep the iTunes library on your iMac or on your Time Capsule. I'd suggest leaving where it is for now, get your network properly. You can always move it at a later date if space becomes an issue.
4. And the other belief I have (true or not?) that the amount of photo space being taken up on the iMac is what's causing slow performance. Any idea if this is the case or not?
Lack of free space on the OSX hard disk can cause slow downs. How the space is used doesn'y really have a bearing on things. However, moving your photo library onto an external disk is often a good way to free up space as they can reach an enormous size easily these days with HD video and high mega-pixel cameras.
It's a reasonably painless move to transfer an aperture photo library to an external disk. I'd suggest using the NAS.
As for slow downs in general:
1. remove any unecessary applications that run all the time/in the back ground/on start up.
2. Use Disk Utility to check disk permissions and verify disk.
3. Consider a tool like Onyx and run the monthly scripts (it's free and works well)
4. Move your photo library to your NAS
5. Ensure you have all updates for OSX and aperture
6. Consider increasing the RAM in your iMac (if you haven't already). It's a pretty cheap and painless upgrade and Aperture will thank you for it.