Reading through the game manual, and can’t find the width from the alliance wall to the line that opponents can’t cross. Am I looking in the wrong spot 
Were you looking in the Arena section?
From 2.2.2:
There is one ZONE for each ALLIANCE, located immediately in front of the ALLIANCE WALL for that ALLIANCE. The ZONE is approximately 18 feet wide and 7 feet deep. There is a 2-inch wide yellow CAUTION LINE located 4 feet in front of the ZONE. The CAUTION LINE is used as a visual warning indicator to the opposing ALLIANCE that their ROBOTS are getting close to the ZONE.
The we can hang out in the warning zone all we want right?
That’s what it sounds like
As long as you don’t cause yourself to cross into the Opponent’s scoring zone, you should be fine. That warning zone, I feel, may be vital to playing good defense.