Turn back now...
Here's my interpretation based on the haunt rules.
It's never stated that the traitor is dead nor does it sound like the traitor is the real enemy it's just one of your party has an alien infection and may be turning into an otherworldly specimen.
Ever since I touched that meteorite, I can feel the change working in
me... If I do not succeed [creating an antidote], perhaps these notes will help whoever comes after. I pray, though, that this disease will die with me.
Personally, if someone dies during the attack against explorers immune system I would think that the explorer is no longer an explorer but in a transitioning state. The book says:
If a traitor dies for any reason... a transformation to the third stage takes place. The transforming traitor skips their next turn.
Since the transforming traitor cannot do anything on their upcoming turn since they're in the process of transforming into the 3rd stage alien, I would assume they're no longer an explorer and would not be able to interact with any of the explorers in the house which includes trading monster tokens.
There's so many haunts that need clarification which unfortunately can't all be verified by the rulebooks. The last page of the rulebook states:
What if there isn't a rule that for x scenario? In such cases, come to
an agreement as a group for what makes the most sense and go with it.
The most important rule is to have fun!