"t:0111000000000000=d:00000111 <--what purpose does that serve?"

That's the fine vertical scrolling register. It allows "smooth" vertical scrolling, analogously to

"x=d:00000111 <-- I can't get even tile-only scrolling so scr** that"

I'd highly recommend you reading Brad Taylor's doc on PPU addressing. It does a better job at explaining how scrolling works.

If no one else posts other comments on your post before I get home then I will
answer the rest of them.

hydesprojects.cjb.net