Hello, I just read this post and as a HD main I will answer with my opinion. The fact HD is different than FI/FL bc the fade out time increase may be a bit annoying (it's for that HD skins exist but I don't use) but in general HD doesn't affect anything on the gameplay. It just allows me to focus more on the notes than if I don't use the mod.
"How is it fair for mania players to have such an unfair advantage while player HD while other gamemodes have it somewhat easy and get more PP for playing with the mod."
In mania, only difficult decrease mods and DT affect the PP because it makes the gameplay easier/harder, HR just reduces the judgement bar so it don't really has inconvenient. In other gamemodes, HR changes the gameplay by spacing the notes and make them more difficult to touch (example with osu!standard)
So, to conclude this post, HD on mania isn't really unfair as you can think. If it was really that unfair, HD players would have already complained about it.
PS : My english is maybe not very good but I think you can understand what I said.