From what I understand of the summary, the GetCurrentDisplaySettings implementation can be inconsistent and relies on figuring out everything the compositor supports by guessing. Have you bugged the Wayland devs about adding some protocols to support viewing the display specifications?
This might go against their design choices, but it's not much of a danger since applications can't self position or control other windows, it would just be asking the compositor what displays are available and what modes they support. Maybe on top of that you could allow choosing a display to output to (rather than exact positioning, it's just a general "I want to be on this display") which would work well for games, as they tend to have display settings there to see what they can do.