Wow! I had no idea he had a pacemaker. My 94 year old father-in-law just had one put in. They are different from the old ones. More like a jelly bean that gets implanted into the heart. When it has to be replaced, they just leave the old on in and implant a new one.
As for EJ getting shot, I think it’s not going to be any of the usual suspects. I think either Arnold or Rachel Blake might do it, or EJ’s assistant who seems to pop in every now and then. I can’t imagine it would be a major character.