Fin rot. You will need to test your water and make sure that it's ammonia, nitrates, and nitrates are at 0, you have filtration, change your water, and then buy it some antibiotics from the fish store.
This is not a Betta question so it belongs elsewhere.
First thing would be removing the fish that is attacking the Beta's fins. If it is not a fish, the Beta may have a disease such as tail rot-in which case you need to isolate the fish and treat it. But there are supplements that can help with the health of the fish.
Genetics.
No only the males. Why because the fins and colorfulness is used to show off and show to the males they are better.
there is 1 reason that i know of is that older male bette's will lose there fins. "Dont wory they'll grow back"
Yes the tail fins will eventually grow back but they will most likely not be as good as they were
Whitebait is a type of fish so it would have fins.
a female beta is rather drab with only a little colouring at the tips of her fins and tail
If you put a Crab with a Betta I can almost guarantee the crab will rip chunks out of the Bettas fins first and end up by killing and eating the Betta.
The Fin regrowth time is Relative to the amount of damage Done, A beta's fins Grow at about the same Rate as a humans finger nails. Hope this helps.
A Tui is a type of bird, so it would have feathers for flight.
Fins up would be evät pystyyn in Finnish.