Honestly wondering what trick the MCU is going to pull to make RDJ being both Dr. Doom and Iron Man make sense. Maybe he’ll just never have his mask off when RDJ is the actor, or maybe it will be multiverse shenanigans. I hope it won’t be like Coulson/Sarge in Agents of SHIELD wherein one is essentially a cosmic clone of the other.
I doubt that as masks disappear for a couple of lines, even in cases where it reveals the secret identity in public.
The age, Doom’s scar, and acting will make it all work. I haven’t seen RDJ do much with his voice though. He always sounds like him.