Cork taint, TCA - trachloroanisole, is a natural compound that can exist in cork. If present, it can throw off the contents of a bottle. Most people associate this with wine but indeed spirits can be affected. Cork is not the only means a product can become subject to TCA but it's generally considered to be the most frequent cause.
If there's a real rarity of this bottle, interested parties may be willing to pay a lot for the item - even if the cork condition isn't ideal and the content quality is in a questionable drinking stage.
When I used to work in the wine auction world, I saw wines all the time that were surely dead from a drinking stand point but had a collector value because of producer, history, vintage ... The right wine could garner thousands of dollars a bottle even though its contents would no longer good vinegar make.
As for your perception of lees in the bottom of the bottle, it is possible, given the age of the bottle, the whisky was not chill-filtered. It is possible the sediment in the bottom could be fatty oils and solids that are a natural part of the whisky. Additionally, they could be small pieces of wood from the barrel. This could be due to the barrel toasting or simply the rough shave from the staves.
If you buy single cask non chill-filtered whisky, you will often find this. Though, where it could be, never have I observed it to be a detriment to the drink.
If you decide to drink it, the decanting suggestion is good. And please let us know how it is.