1. The chances are pretty low. HIV tests are very sensitive. Most of the HIV+ persons test positive after 6 weeks. The chances are, I think, around 1%. Its my guess. I hope someone else will confirm my estimation.
2. 0.2 and 0.24 meas the strength of the signal in the ELISA assay. They are pretty weak: negative. The problem is if they are around 0.99. That is an intermediate result, so the test must be repeated. High values means positive. 0.2 and 0.24 is very low.