The small amount of light illuminating the lawn, coming from Good's porch light, might have made her observation possible.
Check out
State's Exhibit 77 (Warning: somewhat graphic photo of Martin's body).
The video you are so fond of shows the same thing. The porch light has a reflective housing that directs its beam to illuminate the patio, forming a sharp rectangle of light. Good mentioned that in one of his statements.
Assuming it was to any degree easier to see someone who was opposite an active porch light, how do you know Martin or Zimmerman wasn't?
Speaking of David Knechel, I learned something interesting about him. He blogged about the Casey Anthony case, and was involved in the judge recusing himself. He was gung ho for conviction, and he ratted out the judge for seeing him privately and praising the fairness of his coverage.
I think she could just barely see them.
She could see them well enough to know they were there.
I don't buy the theory that either Zimmerman or Martin would have been discouraged by the difficulty of identifying the other at a distance among the huge crowd of other people out there. I have yet to see any evidence for such people.