Yes, players leaving is the biggest problem, unfortunately
For the initial mobs that over-aggro you, I believe you can step off the web ramp, but than make make only a few, if any, steps forward, and there will be exactly one mob rushing to aggro you. You will have to wait for it a little.
Make enough additional steps forward, after you get off the webbed ramp, and one of the spider adds may pull you in the middle of them all, with one of their spider web strings. I believe this is not really a bug.
However it is possible to wait for and taunt the two patrol spiders only, and leave the rest (on the sides) alone. Than you can run up the stairs entrance.
After the few stairs, I also recommend to avoid Charge. It has a chance to make you fall through texture. The same is true if you are pulled by a spider with their web strings: you will have a chance to fall through texture if that happens.
I do not yet understand your trick for Prince Taldaram, but I am curious to get it.
On the last boss, I remember you have to fight the evil images three times in a row. The first time the clones will aggro and attack you. The second and third time the clones will be invisible and you will have to use some AoE ability to run into them and make them visible. I remember on occasion I could see and help some of my real group members fighting clones. But for the first group of evil clones, I believe every group member is on its own and has its own clones to fight with. I think the second and third time you fight the evil clones are the times when you should not move and let the tank aggro them.