Aaaand?

For one, the drawings were done by the orphans (maybe kid herself) and had probably been there for quite some time. The drawings are not the real Crono and Marle, so the paper burning wouldnt kill the two of them. All it proves is that Crone and Marle visited them (or that they visited Crono and Marle), nothing else.

The ghosts who appear with Miguel are from a collapsed dimension, if you listen to what Miguel says. They arent really 'ghosts' either, I believe he said they were just memories or energies or some such thing. Even if they were ghosts, it still doesnt prove that Crono and Marle were at Lucca's house.