1. Eren saw the shot coming and quickly transferred his consciousness, and will transform to heal himself.
2. Eren and Zeke ALREADY touched and Eren used the founding titans scream to alter everyones memories so they think they killed eren. (With Magath and mikasa on the film isayama can cleverly show us that everyone else is being fooled, so this one is most likely to happen)
3. Erens head only just being torn off means he is still alive and will be so for the next minute or so, meaning contact with Zeke can still trigger the founding titan.
4. The titans abilities have always transcended space and time itself, simply knowing this and actually thinking eren is dead makes you an idiot.