By the power of truth, I, while living, have conquered the universe.
It's highly likely Truth was observing the first glimpse of the Portal being uncovered. However, the structure looks nothing like the Portal, even for a small section there aren't any of the 14 monoliths which leads me to believe Truth could still be alive. (Yes, I am aware this is before Halo 3.)
It's not explained how Buck escaped from Reach, but it's likely it will be explained... In Halo Reach...
ODST's epilogue is a month after the drop, Johnson, Miranda, Arbiter, Spark (etc) all made it back within that time - likely due to Rtas when he took the Cruiser back at the end of Halo 2 and gave them a lift.
Johnson escaped Installation 04 on a Pelicn with several others. It's explained in Halo: First Strike.