Can you wear your Marine unit patch as an Army combat patch if you deployed with the Marines? Policy appears to say no.

My understanding the combat patch, can only be worn when forward deployed

Back during the bad old days in Vietnam, Marines with prior combat service were authorized to wear that patch on the right shoulder, and rightfully so. Well earned.

One of my Drill Sergeants wore 1st Marine Div (vietnam)combat patch in 1982.

I can't imagine why this subject is important to so many of you. Marines don't wear combat patches. SEALS don't wear combat patches. That's good enough for me.

The Marine Corps does not have a combat deployment "patch". The Marine Corps does have the Combat Action Ribbon. If you earned it, it will be in your service record and your original DD214. If the Marine Corps did not award it to you and you think you earned it, you can appeal to the Marine Corps for reconsideration. Like the Army CIB, it has specific requirements for earning it, e.g. engaged in close small arms combat with enemy combatants. If you earned the CAR, you wear it on your ribbon bar. Likewise if you earned a Sea Service Medal, Good Conduct Medal or Overseas Deployment Medal: those are specific awards that were earned and awarded.

It doesn't work like that. The combat patch is Awarded by the Army to Soldiers. Even Marine patches can be awarded to Soldiers. But the issue is that you must be a Soldier at the time. I have seen this and the irony of the whole situation is that Marines usually don't want patches or insignias.

Putting on my Remington Raider cap, were you in the USMC when you deployed? No. You were a soldier attached to a USMC unit. Why not wear the patch of your parent Army unit? Your record would reflect that as a member of, say the 82nd Airborne, you were attached to and deployed with the 2d MarDiv into a combat zone. No ambiguity there.
As a Marine, I'm not familiar with, and never particularly cared about the Army uniform regs associated with all the stuff designated for wear on their uniforms, as has been said, our EGA and service ribbon was enough for me, but it seems to me that your individual service recognition shouldn't depend upon your entire parent unit being assigned to a specific theater, or what unit you were attached to from your parent unit.

No.

OMG Rally Point please stop this thread. I don’t care anymore and 3 years later people are still answering and it keeps notifying me over and over.

There are few who are actually authorized per the regulation, but many who just wear it anyway because they switch to Army and don't want to be a slick sleeve. The reg says, some from WW2 and a few from OIF (early OIF) and that's it. All others are not authorized. It's that simple. This shouldn't be that big of an argument. Your pride doesn't matter. Just because AAFES has the patch doesn't mean it's authorized.