A cryptocurrency with demurrage tax would fall directly on holders, and therefore hopefully discourage hoarding. encourage dumping.
there i fix that for you
There will not be any dumping if there was no hoarding in the first place.
(And, as I wrote, that is one of the problems that bitcoin has: so much of bitcoin's mass is hoarded, that the price could crash in a blink if even only 10% of the hoards were to be dumped.)
(as your scenario)
miner will get the coin and dump it, as long as there are buyer exist there is.
edit : unless you want to put one centralized entity which back up the system by buying all coin at certain price and selling at certain price (hello paypall v2.0

).