So long story short is I'm stuck at this part. I have the diary for Saverok and everything set, but its those blasted doppelgangers. I know the solution is to swarm the dukes with Summoned Monsters, but EE only allows it to be set to a max of five creatures out at once....so with their haste they just rush around to the dukes. The dukes not being surrounded go and charge at them to their deaths. I've tried web and entangle. I've also tried killing them before they transform, but that doesn't work too well either. Any solutions are greatly appreciated :D

Confuse, fear, incapacitate. Concentrate on saving Belt. You only need Belt or Liia to survive, but Belt has better survivability, so place your summons around him.

I've read somewhere that you actually can continue the game without either of dukes surviving. You just need to know where to go after the battle is done. I will not say more at this time not to spoil things.

When it comes to the battle in the palace. Hickory already gave you some idea. Remember that you can beef up your party before entering the room (haste, heroic potion, strength potions etc.). Concentrate all your conventional firepower on a single enemy, kill it, then move on.

I also wonder if dispel magic on doppelgangers would work. They cast haste on themselves and become really deadly once they do it. Never tried it myself but it might be worth checking.

I think it does, unless I'm badly mistaken I used Dispel magic to cancel their haste (and yes, that is necessary).
Have never tried it myself, but isn't there a spell ("Oityluke's resilient sphere" or something like that) that makes its target invulvnerable? I think I read you can use that one on the dukes so they won't get killed.

Otiluke's resilient sphere is one of the possible solutions (casting it on one, or both dukes) and basically all the mentioned spells will work great against the doppelgangers. If you have some great melee fighters, you can rush with them (hasted preferably, plus one with boots of speed) between the dukes and the doppelgangers. They sometimes switch target and your guys (or gals) can tank them, while the rest can focus on the ones actually attacking the dukes. This, in a way, is a solution to the limited number of summoned creatures. Dispel magic is probably the best spell to use in this encounter, though.

After you are allowed entry but before you enter the main room:
invisibility (10' radius?) on your party (possibly minus 1). You can then enter the main room without triggering the scene.
Then guard the duke with your tanks and position the others strategically (mage nearer the edge, etc).
That way you've got a head-start.

Then your minus-1 enters (or breaks invisibility by talking to someone) to trigger the scene Or even better - wand of monster-summoning near the dukes to trigger the scene (though this breaks the immersion a bit as the scene carries on as if a bunch of dogs/gnoll didn't just show up around the dukes).

It could be considered meta-gaming, but then you know there are assassins after the dukes and that the big bad guy is there - it'd make sense to make a discreet entrance to secure them.