Well, thanks again for the other ideas, but i finally managed to solve it. Check this out...
The northern path leads to the journal that describes how to make a perception potion in order to find the secret door to Cassandra's grave. Now, to cross that section, i had to teleport one of my characters to a very specific spot in which the lava didn't touched me, and from there teleport again to the lever that deactivates the sentinel. I'm pretty sure it's a very cheesy way to do it, but i honestly coulnd't find another - nevertheless, that path isn't really necessary to advance.
Now, the eastern path is the one that leads to the waypoint, laboratory and secret passage. As i stated before the lava was always there regardless the sentinel spotted me or not, and there weren't any ways to stop it. After trying a million things i teleported the sentinel far away from the lava and what happened is that he started to spot me with the INTRUDER ALERT line, yet the lava dissapeared.
That's right, it was backwards - when i moved my characters away from the sentinel the lava showed up again, and when i stood right in front of him the lava dissapeared. I'm pretty sure that part is somehow glitched, because it also worked ONLY when i moved the sentinel away from the corridor.
So, again, thanks for the help, but i guess this time the solution was rather... obscure. :P
