Luckily Shanks has the Giants on his side because otherwise, he might lose to Kidd, huh...
You got a couple of things wrong... He can have both Lunarian durability and DF durability up at the same time.
However, it wont make him any tougher because Lunarian durability is peak. Adding weaker durability on top of it is useless.
Even in flame off mode, I doubt metal is more durable than Lunarians. If it's the other way around, then flame off might be buffed by DF.
Of course they are. What they are not is durability buffs. Because they dont tank damage with lasers or with blood, fishy...