The only question I can answer with certainty is you don’t need emblems. Heck, I don’t even have a fully leveled 4-star team and I ranked 600 something on legendary. I didn’t even use a single 5-star hero, let alone emblemed ones. edit: you don’t need emblems for rare either. But you do ideally want a good team of high attack heroes like Gato, Namahage, etc. Costumed hawkmoon, costumed brienne, costumed gunnar helps a lot too.
The other questions, it’s really a lot more nuanced, and not as mechanical as the strategy guides make them out to be. Well, maybe it is that mechanical in the top 10 level, but I’m not there.
I know for a fact that for top 1000, you don’t need to be super optimal at all.
And there’s no set time when to use items, and what items to use, whether it’s damage items or mana pots; it all depends on feeling and experience.