This is a common problem in educational environments. Apple has done an excellent job of selling iPads and Macs to students/staff and they want to utilize Airplay/Airprint/other Bonjour functionality. However, as you pointed out, these features rely upon a single broadcast domain for service discovery. Enterprise/educational networks just aren't structured that way.
The problem is so prevalent that many educational institution IT staff got together a few years ago, and petitioned Apple to fix Bonjour to work better in these environments.
To directly address your questions, it will usually require very specialized configurations to accomplish the spanning of Airplay services across your network. And the specific configuraition will depend heavily on your current wireless solution (Cisco, Aerohive, Ubiquity, etc). In general, if you search for your wireless vendor and Bonjour, you should find documentation to at least point you in the right direction.
I have had mixed success deploying Cisco's Avahi Bonjour gateway solution, and wouldn't recommend looking into it unless you absolutely have to.
The bottom line to me is, as you pointed out in your third question, you'll always be at Apple's mercy because this is a closed, proprietary, undocumented service* intended for home network environments. So, unless Apple decides to change that, I would avoid implementing it in an Enterprise network wherever possible.
*The underlying code for mDNSResponder is open, non-proprietary, and available under the Apache license. However, Apple's implementations of this internal to their iDevices and MacOS are all outside your control and could change at anytime.