Vanguard of the Dragon and XYZs

Can Vanguard discard more than one Dragon type to get more than one 300 ATK point boost?

Also, can the XYZs detach Overlay Units during battle? (Like detaching Leviathan Dragon's last Overlay Unit to boost to 3000 when Black Luster Soldier attacks)

Vanguard's effect can be used many times in a turn.
It depends on the effect of the Xyz monster, and Leviathan Dragon cannot detach during Battle Phase.

You can activate "Vanguard of the Dragon's" ATK increase multiple times in the same turn. The ATK increases lasts as long as "Vanguard of the Dragon" remains face-up on the field

Leviathan cant detach during battle phase, only monster like Kachi Kochi Dragon or XYZ Evigishki, Lavalval Ignis, etc could detach duing BP

Xyz Monsters are in no way different than any other monsters, Ignition effects like Leviath Dragon can only be activated during Main Phases, Quick Effect can be activated at any time(except damage step unless they negate), Trigger activate when conditions are met.