I checked your method and yes, using twice "previous item" button to select invisibility potion and then use it is slightly too slow. By the time I'm using potion, I get second alert. After 20 failed attempts I used my method and ghosted this watcher on my first try. So it's possible to do it consistently.
I also heard that there are some functions in Controls added in Newdark. So I wonder is binding invisibility potions was even possible in Olddark?