The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve’s Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.
Depends on the adapter and source. You may find issues when playing HDPC protected content if you buy a low quality adapter.
DisplayPort to HDMI doesn’t need an adapter since DP has an alt signal mode with HDMI support.
Although I’m not sure 2.1 signalling is available yet or not.
DP++ HDMI support tells the GPU to output HDMI. If the GPU can’t output HDMI 2.1 over an HDMI port, it can’t output it over a DisplayPort (as a general rule; you could theoretically wire the DP for a higher standard, but why would you?)