1 and 2 are incredibly dependent upon your situation, and in a vacuum of numbers on paper it looks good, but you're going to have to ask your DM.
Things like terrain come into play here; it depends very heavily on your style of game, but it's not exactly common to fight on 100 feet of flat, open terrain, much less 600, especially if you're underground, where lighting can become another vision issue- since most of the best darkvision effects that I can think of cap out around 120 feet, meaning you also couldn't see them at that range to target them with the spell in the first place.
3) If an enemy is surprised, which is entirely at the DM's discretion, then no- it wouldn't stop being surprised after the first hit from EB.
The DM determines who might be surprised. If neither side tries to be
stealthy, they automatically notice each other. Otherwise, the DM
compares the Dexterity (Stealth) checks of anyone hiding with the
passive Wisdom (Perception) score of each creature on the opposing
side. Any character or monster that doesn't notice a threat is
surprised at the start of the encounter.
If you're surprised, you can't move or take an action on your first
turn of the combat, and you can't take a reaction until that turn
ends. A member of a group can be surprised even if the other members
Any character or monster that doesn't notice a threat is surprised at
the start of the encounter.
Sage Advice elaborates
You can be surprised even if your companions aren't, and you aren't
surprised if even one of your foes fails to catch you unawares.
Remember, there is no 'surprise round.'
Surprise is like a pseudo-condition- it applies at the start of the encounter and applies until the end of the first turn of a surprised creature. While the trigger condition to be surprised is to have not noticed any threat before the start of the encounter, once the encounter has started, it lasts until the end of their first turn, not until they're attacked the first time.