Kurt Angle's WWE contract was bought out early, which is why he became a free agent immediately. He is still due merchandise checks and PPV bonuses, unless he agreed to forgo those for the early release and the severence package he received. In any case, signing with TNA should not drastically affect what WWE owes him, if at all.
The feeling in WWE during the hype about TNA's big surprise was that Chris Benoit was not at risk of leaving. While it's not clear what his contract status is, he is seen as someone who would give proper notice and leave on good terms, assuming he'd even consider leaving.
According to WWE people working with the Ford Field ticket office, if every seat in the building is filled, there would be 78,000 people in the building.