How are they charging $10 for this $3 watch from 1990?

Anything that was $3 in 1990 is now $10. Back in 1990 gas was $1.16/gal. The average home price was under $150000. As time moves on prices go up.

Does the count down timer be set to seconds?

No countdown timer. Look at Model F201WA-1A for that feature.

how far in depth is the water resist

If you want to be absolutely safe, get the Casio MRW200 dive watch which is good for 100 m. Casio F91W is water resistant and you could take it in shower, swimming, etc.

The rubber band broke after 2 years of use. I love this watch and don't want to get a new one. Any tips for a replacement band? Maybe a nylone one?

Aliexpress or ebay you'll find almost exact generic replacements for a couple bucks shipped. Search for words like F91W strap, 18mm watch band. You can also use normal 18mm spring bars instead of the pins that are stock with the F91W.