How do I stop my game from launching as full-screen?  It seems to run much better a a 640x360 embed so I'd like that to be the default mode, with the player pressing the button in the lower right if they want to bring it into full screen?

Shouldn't it be in "edit project -> Embed options -> Embed in page"?

Yeah that seems to be highlighted but it'll still just go into full-screen when it launches

It could be a setting in your software that is forcing the game into full screen.