"Bane, Bhaal and Myrkul inherited most of the portfolios of Jergal when he wearily stepped down from his position and then faded into near-obscurity. The death of those deities left Jergal in the service to Cyric and then Kelemvor; he worked well with Kelemvor, but retained his scorn for Cyric. He spent much of his efforts combating Velsharoon's efforts to prolong life into undeath.
Jergal had a fondness from his time in Netheril as Nakasr for who the once-dead Amaunator was. Their opinions of each other after the Second Sundering and the slight changes to Amaunator's ethos and ideals were unknown."
Whether I want Withers to be Jergal or not (I don't know if I care on this one), keep in mind he is no longer a god. He gave up his godhood to the Dead Three and now serves as a seneschal of Kelemvor. The fact that he has a fondness for Netheril and that Netherese magic is heavily involved with our characters also suggests Jergal.
So, I'm guessing that he likely is Jergal because:
1. "Herein lies the Guardian of Tombs". A title of Jergal
2. He is not a god any longer so God rules don't necessarily apply.
3. The "he" in "so he has spoken" is likely Kelemvor whom he serves as a seneschal.
4. He likes Netherese and there is a lot of Netherese going on in BG3
5. Jergal has a direct connection to Bhaal's story so it would make sense to have him in the final game in the trilogy - assuming it's the final game.
6. The whole crypt is Jergal's, and his statue is right outside the final sarcophagus chamber just like an Egyptian crypt might have a statue of the pharaoh buried within.
Since Jergal's not a god anymore, he could be killed. Right? However, at most PCs should be level 2 at this point, 4 at most if you return to it later. So killing a once-god would be pretty much an impossibility. The DM should just call it impossible - as they have - so players move on.
