Andrewr,
Without radar, it's a bit tricky to assess the exact distance between the two aircraft but i've heard the E190 QAR is very good but even if there's errors with the TCAS you're overlooking that a potential collision was imminent if the crew were not alerted by TCAS and the aircraft being stationary in the windshield.
"The GA-8 was also 500-600 feet higher. 2 miles and 500 feet doesn't seem like something to worry about with visual separation."
This is a little concerning Andrewr. The big picture here is that a jet's airspace was compromised by a light aircraft. This IS something to worry about.