1. Since the spirit in the body is weak and doesn't have much of a sense of time, etc., there won't be much to glean from its memories. He can pick up on the fact that something was wrong prior to the attack in the Necropolis, and to the extent he can hear lyrium, he may be able to hear something faintly similar to red lyrium.

2. Removing the spirit from the corpse will make the spirit chill out somewhat over time, if it can be contained, while any new spirit put into the corpse will freak out.