Although you can run the latest 1.8.5 on the UDM, Ubiquiti is only offering this update for the UDM Pro. As you pointed out, the UDM says 1.5.6 is the latest version. I have stuck with 1.5.6 for my customers with UDMs. My thoughts are wait till Ubiquiti makes this an offered upgrade for the UDM.
Yes. For the time being, Ubiquiti is not offering the new 1.8.5 for the UDM. That newer version of the firmware is packaged with the 6.x version of the SDN Controller. We still get periodic updates for any attached switches or access points. I can only presume that Ubiquiti wants further testing.