What you're doing is definitely above and beyond the average home user and you're starting to tread outside of the Apple realm where "It Just Works" and you need to have at least a basic understanding of the underlying tech or you will end up with a lot of frustration as you try to get it right. If you want to do the, I would start reading up on OS X Server, Network User accounts and Profile Manager to beef up your knowledge of these systems. Chapter 4 of this PDF from Apple is a pretty good primer on where you're venturing:
http://training.apple.com/pdf/mac_management_basics_10.9.pdf
I'll touch on some of your other questions:
The Server processes will run the background. Most of the services are already in the base version of OS X running, the Server app just turns on some options and gives you a GUI with more capability to easily configure them.
You can pre-configure a lot of settings for your users in Profile Manager. Some will say setting up profiles for just a family of users is overkill. It is just my wife and I at home, but we have an iMac, a Hackintosh desktop, a MacBook Pro, two iPhones and two iPads. Being able to just push all of our settings to a new device or after a clean install/restore wherever we are is really nice.
For your MBPr account, yes, I think I would recreate it as a network account on the iMac when you get it. You'll want to configure it as a mobile account in Profile Manager so that a local copy of all the user data is saved on the MBP and not just accessed over the network. You can also do this with any other accounts that might want to login to the MBP. You setup Home Directory sync to your preference. Mine is setup for at login and logout. You can also set it up to sync in the background every x minutes. This is all configured in Profile Manager.
Being logged in to your account on your MBPr will not stop anyone else from logging in locally on the iMac. For simplicity's sake, though, you should try to avoid being logged into the same account on both the iMac and MBPr at the same time so you don't end up with a bunch of sync discrepancies.
I'm afraid I don't know much about WiFi sync/backup between iOS and iTunes. I backup to iCloud and use Google drive to move data between my devices. I can't be much help to you there, but from my basic understanding, iTunes must be running in the target user account to sync over wifi.