SEPM 14 managing 12.1 clients means they can download content so I guess I'm lost on your comments. All of my 12.1 clients (thousands) are reporting to a 14 SEPM and getting content from it. Otherwise, I'd have some major issues on my hands. No need for me to run dual environments.
This is what needs to be configured.
Hopefully someone else chimes in who knows because clearly I don't. Or just open a support case,