Do anime videos have a stock of voice-actors words?

I don’t think so, but I could be wrong!!! In any of the “behind the scenes” that I’ve seen, it just shows the actor reading the words in sync to the character on screen. There is NO mention of what happens IF the actor dies. I’m going to assume that it is treated much the same way as when a movie star dies while making a movie. When Paul Walker died back in 2013, they completed the movie using his 2 brothers as body doubles with NO close up full-face shots. There are people who have trained themselves to sound almost spot on for popular actors & maybe they are used for the final few scenes. Maybe our resident film expert will know better as to what happens!!!

For physical stand ins, it’s common to use a sibling. @LadyMarissa‘s comments reminded me of Lana Wood filling in for her sister Natalie during a couple scenes in Brainstorm.
Voices are different, though.
While there may be unused stock dialog, it becomes quite tricky to match the tone, key, and temperament desired.
Usually, they find someone to impersonate the voice. I recall they used Rich Little to finish a voice over nearly completed by another (possibly Johnny Carson).

@RedDeerGuy1 You might be interested in this AI Audio Deepfakes as shown on YT. They use their everyday studio voice over artist, put them on a special AI machine & come out with some amazingly good soundalikes!!! It still needs some work as they don’t sound exactly as they should. As @filmfann pointed out, it is very difficult to fool the audience with voices!!! I’ve seen Jimmy Fallon mess with clips of Obama & have him saying & doing things that make NO sense!!! So, it can be done with the right person. Jimmy Kimmel has a guy that sounds very close to the My Pillow guy even without the technology. With it, I bet he’d be spot on!!!

That link to the Deepfake is about 20 min long & @RedDeerGuy1 might be the only one who cares enough to watch it. I didn’t make it all the way through the first time I tried to watch it. It does get better as you work your way through it though!!!

Doubtful. The odds of a voice actor kicking the bucket during any production is almost non-existent. And keeping such an array of wording would be WAY expensive, so not worth it. Might as well get another actor and redo the recording.