1_ When you're playing as John in a level where there's supposed to be a sprite of the OC you're playing as, he'll be replaced by Zaxtor's sprite instead :

2_ It appears that if you're using alternate shells with his Shotgun then you will be able to fire that type of shell infinitely ! I was in the Shoping Center (backed up from Ep1) and I had maximal normal ammo when I started using the explosive ammo you find in the level, I've been able to use them and still have 10 of those till the end.
Also reminding that John's slot 8 rocket launcher can be picked up but not the ammo for it.