Hmm, offering suggestions - What the hey, I'll take the plunge.
[I don't play Evil, so I can't offer anything on the "siding with Bodhi" angle.]
One of the things you are doing for Aran, with regards to Mook, is protecting a shipment of equipment to be used by the ST in their fight with the Vampires. So obviously this must be pretty good stuff, focused on fighting the Undead.
One of the lamest bits of cheese in the game currently is that you can get Daystar without a fight, as long as you have someone who can pick a lock - this always bugged me [although I have to admit I often take the sword anyway - it doesn't bug me enough to NOT take it occasionally...]
Following the death of Mook, you scare off Lassal, but nothing [really] is ever heard about the shipment - Aran gives you some stakes at the appropriate time, but nothing else. This pretty anti-climatic, to say the least.
So.... what about making the shipment a real event? It would include Daystar, the stakes Aran won't just drop on you, and perhaps some Holy water. [I for one would LOVE to be able to upgrade Daystar BEFORE chapter 6, with the IU mod - this could be a way.] Anything else appropriate for fighting the Undead would be good - the shipment is for exactly that purpose. Perhaps a scroll or two, Hold Undead, comes to mind.....
After you "lose" both Mook and Lassal, Aran would need you to track down the shipment. Not knowing where the lair of the Vamps is, you would have to continue on to the "traitors" quest to figure that out.
Iin addition to the location of the rival guild, Gracen could also have a note detailing, or hinting, where the shipment ended up. Perhaps your scaring off of Lassal interfered with the Vamps getting it back to the tombs - perhaps daylight came and the Vamps had to abandon it somewhere expecting to get it back later....
Now as to the location of the shipment. I don't know how difficult what I might suggest would be, of course that would control whether or not some, or any, of my ideas are out of whack with modding capabilities....
My first choice would be on a ship. Either a copy of the one in the slums, or one from BG 1. The party has to locate the ship [perhaps by talking to sailors/pirates at the docks] and then get to it, enter it, defeat the enemies [Undead of course - the Vamps killed all of the crew so these could be re-animated as something tougher than basic skeletons or zombies] and find the stash. Getting to the ship could be a challange, perhaps an early meeting with Sahuagin [themselves investigating the shipwreck], Laceadons, Sea Trolls, etc. Return to Aran and discover that YOU are meant to use the items and deal with the rival guild....
Another location could be placed in the Bridge district, similar to the Tanner's house, with a basement opening onto the river and thus the sea. Same type of enemies, and perhaps a stakable Vampire [a lesser one] who you have to stake, to get a key from their corpse to get the shipment. Perhaps stakes are more than just pieces of wood, perhaps they have been blessed/sanctified such that getting one should be important. IIRC, the Gnome Merchant in Trademeet sells one [which is pretty useless] as do some temples - so perhaps a clue or two indicating that before you head out to the shipment's location, you should get/buy one.... Or Aran could give you ONE, perhaps mentioning that its the only one he has...
After recovering the shipment, the party is ambushed somewhere on the way back to Aran. The Vamps have discovered what has happened and make some effort to intercept the weapons shipment before it can get into the hands of the ST. Especially if the shipment is held in a new area, the travel time from there, to Aran, can be adjusted such that the party ALWAYS arrives somewhere, on the way back, at night time, to face more desperate Undead. This can be a somewhat tough battle, since they now have Daystar and whatever else they may have recovered from the shipment...
Making it appear that it is a timed quest might be fun, i.e. that the party has to locate and recover the shipment before the Vamps can get it back to their lair - a little tension, a sense of watching the clock....
It would make winning Daystar something better than sending Yoshimo in to steal it immediately after meeting Gaelen Bayle - and going back for the Lich in Chapter 6 [or whatever]. Give the Lich something else to guard to make up for him "losing" Daystar [it needn't be an item after all, some gems, jewelry or cash, perhaps].
Anyway, that's what I got off the top of my head. A valuable prize to be won [by actually fighting for it], perhaps some other trinkets [like the holy water] at least one good fight [and probably more] and it makes some sense [to me anyway].
With the oceanic aspects, its an opportunity to use different monsters, and perhaps even another sea voyage [with a cut scene etc]. It comes with lots of opportunities for more Undead encounters, "turned" STs and who knows, another "unwinnable" fight with Lassal, to really get the party motivated to take out the creep.
Well, I hope that's food for thought anyway.