The first thing to establish is that the missing kama is proof enough that the body is fake. I don’t see why people don’t understand that. A minor detail is usually all the proof you need. Imagine an evil rogue that is harassing your party uses a disguise check to disguise himself has one of your party members. You make spot check to notice that your friend has blue eyes when you know his eyes to be brown. Would you say that’s just a minor detail that you should ignore?
Now that I have established my belief that the missing kama is proof of a fake body, let me establish why a Will save does not make us ignore that proof. Imagine your talking to a German fellow and during the conversation his accent slips and you hear him speak with his native English accent. He then waves his hand through the air like Obi-Wan and says, “You didn’t just hear that.” Of course you heard him. You caught him slip and now you know he is a fake.
When we opened the coffin and saw the Kama was missing, and we knew the body was a fake. The same way you would know that your now blue-eyed friend is an imposter. The Will save in this situation is the vampire doing the jedi mind trick and saying, “You don’t see a missing kama.” But since illusions aren’t mind affecting, it cant convince me that I didn’t notice a clue that’s right in front of my face. I’ve already noticed the missing kama and know the body is a fake. The illusion spell can attempt to “jedi mind trick” me all it wants. I already know the truth.
No - the missing kama was what led you to believe the body was fake. It is not absolute proof as you insist, there are other possibilities.
The 2 failed saving throws led you to believe that you were mistaken and that there must be some other reason for the kama not being there or that the kama orginal seen might have been fake.
None of these possibilites was taken because the players felt the body was fake, the players didn't fail a saving throw to determine how they interpreted what they saw. The PCs did make and fail 2 saving throws to determine how they interpreted what they saw and yet still acted as if they never made the attempts in the first place.
Also how did the PCs know the kama was "magical".
Was a detect magic done on it earlier?
Did it glow? - not necessarily proof, but an indicator - a light spell or the equivalent could have been cast on it.
Did the DM tell you it was?
Did they figure it out because of the damage it was doing?