Yeas, the extended funeral scene did imply that tadashi was alive somehow with no memory of him, his family, or even that he almost died, but that was a teaser. They did it for fun. and burial lots do hold those even if the body was not found. it happens very often. so technically it may have not had a body. but the scene was just for fun and was not connected to the actual movie, unfortunately.
judging by the fact that the building exploded (sorry if this is gross) he probably turned into dust! the building was pretty big so it must of taken a long time to put out. I just wonder how it caught on fire? but with all the technology there, it was possible he hid in some kind of technological fire retardant. but that would indeed take away from the plot. like I said, extended funeral scene was just a teaser. it did not connect or was supposed to connect to the movie in any way. the title "Extended Funeral Scene" was just to throw you off. its not extended. its fake. :'(