1/ No. We don't backport new functionality. The stable series only get serious bug fixes and occasionally hardware-enablement which just requires adding a device id to a list. Various distros base their enterprise kernels on the stable releases and they backport functionality as required for business reasons, but these backports do not get into the kernel.org kernels.
2/ Yes, mdadm-3.1 should work with old kernels (and newer kernels). Obviously functionality that is not present in older kernels cannot be used, but mdadm should fail gracefully when you try to use that functionality.