Firstly, welcome to the board!
As to your questions:
1) I'd play Lerzza as you see fit. A light crossbow would be more in keeping with the monster, and I used Shield of Faith even though she doesn't know it.
2) I made the "lines" between the pillers, both north-south and east-west. This room is both dangerous and confusing. My players got frustrated with it, which made it even worse since they got a bit wreckless.
3) You are correct, those not seeing Tsathzar Rho can not affect him-by the rules-but feel free to alter this as needed. It's your game!
As a bit of shameless promotion, here is a link to my recounting of me and mine's venture through this great adventure:viewtopic.php?f=20&t=2568
Good luck with your adventure!