Who even lets betta fish fight anyway? If you don't want them alive, just sell them to someone, sure they will survive but you will never see that betta again. Letting two bettas fight to death is like giving a gun to a cow and telling it to shoot another cow.
On the "do shrimps eat algae", most species of shrimp eat algae.
I'm not sure about the what do you do with the tank during the meantime part.
And try to add a space where the shrimp can hide from all other fish, I believe the reason your shrimp were slowly going missing overtime was because...
None. I love all the fish I buy!
Except that frog but its not a fish.
I don't even know what it was but it looked cool so I bought it and then it suffocated itself, at least that's what I remember, it was long ago.
Yes, sharks are quite terrifying, but they aren't going to straight up eat a boat for breakfast or something, if you stay a distance from them and not anger them they can be cuddly pets.
This video: