It sounds like you have it connected properly. That optical port on the Soundbar is an input, so you can't connect the Oticon adapter to it. I don't know if the TV would support sending audio to both its optical output as well as the ARC, so your connections sound right to me. I can't understand why the Oticon app wouldn't work to control its volume, so something seems wrong there. You might want to take the Soundbar out of the picture and just have the TV set up for SPDIF, connect it to the Oticon adapter, and then see if the Oticon app works. I wouldn't expect there to be any difference (a splitter shouldn't affect anything), but if that doesn't work then maybe you can take it up with Oticon and ask them what the problem is.
As for the Roku remote, if you set it up to control TV functions (power, volume), then it's sending a signal to the TV for the volume buttons and that only works for the TV's speakers or a device connected to the TV that supports CEC (Consumer Electronics Control). Different manufacturers call it different things and I think Samsung calls it "Device Connect" or something like that. I'm far from an expert when it comes to CEC (AvsGunnar ? ), but I think you either need to "unprogram" the Roku remote so the remote goes back to controlling the Soundbar's volume (you'd lose the TV on/off function) or refresh the Samsung's connected devices (I have no idea how to do that). I have an old Samsung TV here, but I don't know if it supports CEC. If it does I'll do some experiments with my Streambar.
If you never set up the remote to control the TV then none of that applies and the remote should be controlling the Soundbar's volume. I have a Streambar and not a Roku Soundbar, so my experience comes from that. I don't think there's a difference in how the connections work and how the remote functions.