What you are seeing on your TV is the StreamPlayer login after booting up. If you hook up a keyboard, you can actually log into the StreamPlayer using your TV as a monitor. Of course, there is no need to do so unless you are doing maintenance on the OS. But this is normal. And after some period of inactivity (not logging in), the screen will go blank and/or the cursor will stop flashing. This also is normal behavior. It is not an issue.
The StreamPlayer does not use a NUC. It uses a standard motherboard.
As for the power supply, I don't think anyone would make a two-prong power supply in this day and age as they likely could not get UL approval. If a cheater plug solves the noise problem, I don't think you'll have any problems using it. But you do likely have a ground loop situation.
Are the Oppo, the receiver, the TV and the StreamPlayer plugged into the same physical outlet? Are you using a surge protector?
As for the HDMI dropping its connection, this is a non-standard use of the StreamPlayer. The StreamPlayer was designed to be used with USB, S/PDIF or both. While it is certainly possible to enable the HDMI outputs in Roon, this is not necessarily a supported use. There are simply too many variables involved here when things don't work out. It may be an issue with the the HDMI cabling, the Oppo, the receiver, the TV or any number of other things. HDMI is a basically a video interface with copy protection. There could be issues with the Intel graphics circuits or graphics drivers not recognizing the Oppo as an HDCP (digital copy protection) compliant device. Without having all the associated devices in hand (TV, Oppo and StreamPlayer), troubleshooting the issue is akin to target shooting in the dark.
We have purposely kept things simple with the StreamPlayer both to keep things stable and to minimize the support time required to keep them up and running. When we introduced the very first StreamPlayer, we had a version without internal storage. But we found that we were spending a great deal of time sorting out customers network issues. I spent two full days one time trying to figure out how in the world one of our customers could have over 1,000 devices attached to his router. In the end, a new router solved the issue. Although you could certainly use a StreamPlayer to play music files stored on a NAS or other device, we now include internal storage in all StreamPlayers to keep things simple. Having internal music storage pretty much eliminated support calls since we no longer have to help resolve network communications issues between the StreamPlayer and a NAS or computer.
I have used a customized version of the StreamPlayer as a media server in my home theater and have had not issues with drop outs or the HDMI signal going to sleep. But I don't use that server for music via HDMI (although I do play concert videos quite often and have had no issue).
Solving issues like these is actually quite easy, once you've identified the cause. The best way to troubleshoot is not to search for the issue or assume any component is at fault, but to eliminate things that are not the issue. The more potential issues you can eliminate, the smaller the list becomes until you eventually narrow in on the cause. In all the years we have been producing StreamPlayers, we have had to sort out a fair number of issues. I don't recall a single case where the StreamPlayer was at fault once it was set up and operating properly. Of course, you can't rule anything out. But it is a very simple device and there really isn't much that can go wrong.
If I get a chance in the next week or so, I will investigate HDMI issues where multi-channel music is concerned. In just a quick look today, I noticed quite a few individuals trying to address similar issues. But I did not run across any solutions. So more research time is in order on this one.