It's correct and as intended. "Least impractical" denotes "most practical" but the double negative, which you accurately note is unusual but which is grammatical in this usage, calls attention to the specific connotation that all options are bad and this, though also bad, is nonetheless the most potentially serviceable of the lot.

We all know that language contains a lot of subtleties, but it is always interesting when someone breaks down exactly how those are used in some interesting prose.

And the prose itself is good too.

I've been an eager student of grammatical nuance in English since my introduction to the written language at age two. I'm always happy to take apart an example of same and show its workings!

I had to read that twice, but it’s correct I think. It’s an impractical route, but all the routes are… the Drake Passage was the least impractical of all of the routes.

Although I did wonder later on why they couldn't have gone through the Straits of Magellan on the journey west. Presumably it was even more difficult for large ships to navigate that narrow and presumably shallow passage.

At that time, the Straits of Magellan were “controlled” by Spain and it would have been extremely difficult to sail through undetected.

Plus, it’s like sailing through a maze where if you touch the wall you die. You really should have someone that’s been through it many times before. Even in modern times, they require local pilots to guide ships.

It was impractical ;)

Or at least, more impractical :)

At most.

* brain explodes *

I read "least impractical" as meaning "least worst" -- the best bad option.

No. It was a best out of a bunch of bad options.

It was the best (of the worst)