Thomas is who he is. You can't begrudge a man for playing his character.
But it's possible the low level henchmen of the big bad don't know where the castle is right now. It floats around. I think we'll need something that can detect it like a diving rod. Also, the casting cleric needs to speak the language of the deceased. Not sure if those things speak Common.
Next time we meet them, we should keep one alive for questioning. For now, since we have no leads on the giant's whereabouts, we should probably hoof it to the tower and see if we can find out anything there. Once we discover the giant's location, we can fly there under our own power if need be, and use the cool polymorph/charm idea at that point.
If Halo wants to talk Thomas into changing his mind regarding the treatment of the dead, go for it in character. Blodget can't do it but you're a holy paladin with a high charisma. Use honey instead of vinegar this time.
I wouldn't fight pride with pride in dealing with Thomas.
Failing that, one of the good birdman priests asked us several times if they could assist us. Sweet talk
him into it. You've got a good case. Time is of the essence. What if the giant attacks the birdman city tonight while we're walking to the Tower? We're the only thing that staved him off last time. Once we blow town, this place is wide open for an attack. We need intel now to go directly to the giant, for the sake of the birdman city.
Good luck!