First, my problem with the Roon remote seems to have been the Energy Saver setting. When I changed that, the Roon remote picked up my wireless network with no problem. This problem had nothing to do with Airplay.
On the other hand, Marantz tech services says the AV PrePro AV8802A only supports Airplay over the Internet connection, and that as has been often correctly said in this thread, is limited to 44.1/16 bit quality. That is a shame given services like Tidal AND/OR in my case connecting into the Marantz through hard-wired Ethernet from another room
However, he says the USB port using a USB stick will support up to 192/24 bit quality, something I have not yet tested. Ditto for Bluetooth from an iPad for example, which sounds a little more dubious, and which I have not tested yet either.
Of course it’s possible to connect a computer directly to the Marantz through a device that conditions the USB signal–since USB is REALLY not designed for high-quality music. Such a device would be, for example, the Alpha USB from Berkeley Audio Design. The Marantz could handle the DAC chores for up to a 192/24 bit stream Marantz tech support says, but an even better choice would be to pass the Alpha USB digital stream to a high quality DAC such as the Alpha DAC Series 2 (or for those with DEEP pockets) through the Alpha DAC Reference Series or some other high-quality DAC.
Thanks for all the thoughtful replies, guys. I think I even learned something!