Jergal was never gone, he's been working for Myrkul, then Cyric and now for Kelemvor as scribe of the dead recording everything that dies since he passed most of his power to the Three. I'm not sure why he would be punished now for it either, considering how long ago that happened.
Also, he's pretty busy recording things, so I doubt the skeleton is him. The skeleton could be working for Jergal thought.
That's my guess as well. The dialogue mentions the power of a chosen which would fit. As would an old avatar. If it were Jergal himself he would be disguising his form and faking his time in the tomb (Jergal looks like a praying mantis.)
