Today I saw a chaplain with a long tab and a combat action badge. Is there a scenario where this is possible?

For a CAB you only have to be within 50ft of enemy rounds. You don't have to have fired back. He could have been in a convoy that got hit or close to incoming morter/rocket rounds. Never seen a Chaplin with one before though.

I had a chaplain in my unit that had prior service as an enlisted man in Viet Nam before attending a seminar and then being commissioned as a chaplain in the Army.

Many people f the TRADOC schools use a similar patch design, the diamond shape a torch and somethings else.

It is possible to have the CAB and the Cross

Did you go ask him. Most of the time someone will tell you there story, all's you have to do is ask.

I know a Chaplain who was enlisted Infantry and then became a Chaplain. He not only holds a CIB, but a TUS Badge. Very possible!

He has to be prior service.

If I'm not mistaken there are 8 Chaplains with an MOH. I see no problem with it.