An HQPlayer instance can only be controlled by one external piece of software at a time. This is an HQPlayer limitation. If you attempt to initiate control from another piece of software, Roon will lose control of HQPlayer.
Does this mean that embedded HQP running on Linux cannot both be controlled from a tablet app and receive input from Roon ?
HQPlayer Embedded cannot be used with Roon at the moment, because the control interface is different. At some point I may look into adding same control interface HQPlayer Desktop has also to the Embedded version.
Overall regarding the control input, there can be only one controller at a time. But this is not related to the content source. The content source doesn’t have to be the same as the control source, the two are independent.