Ker Nethalas - poison and first domain questions

Grabbed Ker Nethalas not long ago, and am about to dive in for a first run. I have a few very basic questions though, mostly related to poison:

1. How does Poisoned work? The condition description mentions that it is cumulative, and can be resisted each turn. I can't find how the cumulative part works however. So my understanding would be that a Poisoned(X) creature makes an Endurance check each turn, remains Poisoned(X) and takes X poison damage on a success (with the damage table converting that to actual damage as usual), or becomes Poisoned(X+1) an takes X+1 poison damage on a failure? Or maybe it always stay at Poisoned(X) and the Endurance check only determines if the damage applies this turn or not?

2. The Duskblade Tier 1 ability makes a target become Poisoned(1). Does the target get an Endurance check to resist the effect applying, or is this specific ability always poisoning the foe?

3. More poison questions: if my enemy is already Poisoned(X) and I apply Poisoned(1) to it, like with Duskblade Tier 1, does it become Poisoned(X+1)?

4. Page 225, the pre-mapped First Domain, is there an obvious room/corridor sections split for the map, or do I just make my own guess? I presume the map was designed with a number of rooms/corridors in mind to make it a reasonable source of loot/experience so I'm a bit wary of splitting it into too many or not enough sections.

Thanks in advance for any insights. I can make my own guesses and roll with it, but since the rules have likely been written and balanced with a specific interpretation in mind, I'd rather go with that at first.