Hi,
Looks like you're learning already! ;)
I have limited experience with VPNs so I can't really advise you on the details. One thing I will say is that you won't max out a 35/35Mbps connection using VPN traffic with an Atom. However as you say you have a much smaller pipe at the other end it shouldn't be a problem. Read this post for some detailed test results.
Is all of this possible without pulling my hair out?
You'll probably loose some hair. ::) It is possible.
There are plenty of people using Realtek NICs with no problems at all. Test it and see. Do you know which realtek chipset it is?
Does your Atom board have a mini-pci or mini-pci express slot? Many do. You could use it as an access point directly, assuming it's located somewhere centrally.
From what I’m gathering, it seems that VLANs are basically the same thing as what I want to do with these virtual interfaces, just on a very small scale. Is that accurate?
You can use Vlans to get extra virtual interfaces no problem. Yes.
I've never used the RB250GS but VLANs are surprisingly straight forward.
It sounds to me like you are asking all the right questions and have a good idea of what you're getting into.
I would try using the Reatek NIC first (you'll probably have no trouble) and get a managed switch if you need it. Remember to go one step at a time testing as you go. A lot of people run into trouble when they swap their current router for pfSense and try to match the previous configuration setting up every feature in one go. Have fun! :)
Steve