Tremmorsense shows no obstructions in the spiral stairs as far as you can "see". The winch platforms are both at the top under the slabs.

The metal is of the size and weight to be "Daral" or as humans would put it "coin quality" (and indeed has the mine's stamp on the base to signify this). Each bar is worth 1sp to a dwarven merchant, because dwarven law set the price for a bar of this size and quality at 1sp. Humans have the odd idea that dwarves trade in metal bars rather than coin, which is silly (imagine trying to buy anything of value) but it is true that metal is graded like this to allow for easy exchange. There is no reason it would be left behind, even if the miners had cause to doubt their assayer they could just get it re-certified or melt the lot down and re-cast it.