Those ARE floor slabs the door was found in the open trench as far as I can read, Taphoi assumed that it had been floored over but that is a reconstruction rather than how it was found, which was as shown in the drawings, It is a bit unclear in the reporting though and we can only hope that they extract their digits and clear these matters up.
On the BNE or 'granular convection'; I have to agree with Xenophon that the mechanics are still not understood, whilst disagreeing with his linguistic asides; 'negative buoyancy' is a common English term, in use at least since the sixties when the actor Kenneth Moore appeared on Parkinson (a BBC chatshow) and described how he suffered from it, but was forever being cast in naval roles which meant he had to swim with some poor stunt man holding him to the surface! 'Least worst' is different from best as it informs the reader that all the choices available are bad.
Back in bumble-bee territory just experiment; take a fairly heavy flatish object ( I used a 20mm cast white-metal casualty figure which had a nice flat base but good weight) and place irregular side down in an empty clean and dry food tub or such, then fill to about 10mm below rim with a granulated substance, I used washing powder. Tap the bottom quite gently and you will observe that the object rises in the matrix. It rises from one edge, however not like a magician's beautiful assistant. This is what we see re the doors. This also requires less energy, and increasingly less energy; if you lift a beam with one end on the floor you begin by lifting only half its weight and then, as it approaches the vertical, progressively less.
Greece has about ten earthquakes a century of a magnitude of 5 or more, each with an incremental effect. The retaining walls are subject to the same effect but being close fitting they are not subject to the same freedom of movement and the finer grains will fill small cracks without being able to lift the blocks, the structural walls and roof of the chamber do show damage which is almost certainly from seismic action, but the shocks do not have to be of the catastrophic order posited by Taphoi to move objects within the fill.
This has to remain purely an opinion, of course, until the position of the finds is actually disclosed. I suspect that there are several stages to the monument's history the original construction and burial between 323 and 250 (nothing narrows the range further and it may be later), followed by abandonment with possible relocation of the entombed following a great earthquake - a period of later re-use of the third chamber for prestige burial in late Roman times, just as is seen at the nearby hill 133(?) tomb; it must be prestigious because someone tore up the peribolos to create sealing walls and filled the tomb to stop ingress which must represent the final stage since the first wall with the sphinxes had not been breached. Third century Roman Greece is another historical gap akin to mid third century BC Macedon. One might point to the lack of grave goods yet the elaborate sealing... whatever was there entombed was never meant to escape, the so-called knife wounds to one skeleton might just as easily be a stake!
I jest, but there have been found ritually killed child corpses in tandem with dogs just outside Rome which seem to be counter evil spirit burials, probably in reaction to a malarial outbreak which the populace did not understand; that was late fourth century though. All the same the Strymon was likely malarial. Antigonid Royal Mausoleum to crepuscular Christian crypt of proto-Drakula.. anyone got the phone number of the Greek tourist board? Payment in olives and retsina gladly accepted
When you think about, it free-choice is the only possible option.