Thanks for explaining what happened. It's good to see that you fixed the problem with the fake MPV installer being downloaded.
I agree that Smplayer shouldn't favor one software fork of Mplayer over another. So I think your method of adding MPV support as an addon is the most flexible option. This allows users who prefer to use Mplayer or Mplayer2, to continue using SMPlayer normally.
I hope you won't let the MPV developers' attitude stop you from supporting MPV. Regardless of their opinion, I think Smplayer is the best option for using all of the software forks of Mplayer, because of Smplayer's GUI customization and advanced features. (ex: keyboard shortcut export, etc) So if any features of MPV aren't currently available, I think it's worth waiting for Smplayer to include those features.
Keep up the good work on Smplayer.