What was up with the blonde guy's scary face at Armin?! Didn't get that scene at all, lol.
It was to show her potential distrust towards him.

He should be throwing nonstop, especially at key targets like Pieck.
Throwing uses energy, and he needs to priorities throwing at people going for Eren. When you're throwing at someone, you're not looking out for the others.

And Eren needs to stop his zombie-like walk forwards and just deal with Rheiner and the smaller titan first. Not sure why Eren is walking so slowly, either, he just needed to regenerate fully.
He's tired and doesn't have enough energy to regenerate properly. He did his Warhammer Titan Spike move many times to save his ass.

I assume Eren wants to eat Zeke to obtain his power, eh?
Eren needs to touch Zeke. The whole point is that they have a Royal Blood person who can use the Founding Titan power (or vice versa). When a Royal is ALSO the Founding Titan, they get brainwashed by the Sage/King. Eren eating Beast just becomes Eren with a another titan power, he loses out on the Royal aspect.

Theoretically if Royal Blood is all that's required he can use Historia, but I don't know if their plan requires the Royal to know what they're doing (or cooperate).