Your question first: automated Chrome at an emulated 375x812 viewport with an Android user agent. A viewport test like yours, not a physical phone — worth weighing that against everything below.
I re-measured the game itself at that size just now, and the cut-off I reported is gone. With How to play collapsed your content ends at 713px, so it sits inside 812 with room to spare. Open How to play and the document grows to 953px, and it scrolls to the bottom correctly — max scroll is 141px and it reaches it. So yes, scrolling inside the game works.
I can also confirm both changes are live from outside: the game frame now carries start_maximized, and your tags went from 1 to 5.
One limit I should state rather than paper over: I could not test the fullscreen launch itself. requestFullscreen needs a real user gesture and my setup cannot produce one, so I verified the game, not the maximize path.
If you can borrow an iPhone, that is the case I would check — Safari there has historically not supported fullscreen on non-video elements, and that is the setting you just picked.