I think the notes are fairly cryptic, but give you hints on a few of the puzzles you need to get through to get the talismans.
As far as the note about the skulls goes:
spoiler:It might not be mentioned in the Ramises note because incinerating the skulls is what gives you access to the other tombs after you get the talisman of Ramises..so it might be in another note about getting into the other tombs. At any rate, once you incinerate the skulls in the lava, it basically opens up the rest of the tombs for you to explore, so you won't have to worry about finding a way to leave the tomb area until you've gotten all of the talismans..
As for the force field:
spoiler:I think it's supposed to disappear once you have the talismans, but it's a bit buggy and never worked for me. I always had to try to get back up the ladder using boulders..tough, but not impossible..