Anyone remember when that tire was last topped up with air, If nobody can remember doing so, then it's not air in there as it would have leaked through the innertube quite a bit by now and that tire should be soft enough to find out that it's air in there.
As for the valve slipping into the rim's valve hole and dissapearing between the tire and tube, I think it will take quite a bit of force and time to get it to move that far by just riding the wheel on a bike so you do not see any sign of it from the valve hole....highly unlikely that happened then.......Unless that wheel is haunted and cursed or something........ I still vote for some sort of solid foam " flatless inner tube" of some sort.....

Chombi