Definately not 100 years old, and certainly less than 80 years; Bottles of White Horse manufactured back in the early 1920's and before were marked "Mackie & Coy" as distributors, rather then White Horse Distillers LTD.
(See Attached Photo for example of a 1921 label.)
The Spring-cap used on your bottle was indeed invented in the late 1920's, and was patented in the UK in 1928, and remained a popular closure for whisky bottles through the late 1950's.
As for the drinkability this bottle, it should be fine to do so; Though you should understand that whisky does not "age" once bottled, even after 100 years it would still only be an 8 year old whisky.
However, the whisky distilling processes used back at that point in time often rendered whisky that is prefered by many to the whiskies produced today...Hence one reason for the increase in it's value.
As for your mother wishing to sell it, Similar White horse bottles have brought a wide variety of prices at auction of the past few years...
Auction result would depend on the number of similar bottles available at the time of the auction, as well as seller interests at the time:
On average the selling price tends result in about $150-$200 USD to the seller after the auction fees are settled.
As for my own collection, depending on the fill-level of the contents, I would spend about $200-250 for a similar bottle...
(Honestly, I would love to have this one in my collection, simply for the unique capsule version it bears.)
Walter C Hurst