You could've returned the "algae eater" (I'm suspecting CAE, but meh, don't know too much about those) and laughed as you returned it if you really wanted to feel better. I've been told flushing a fish down a toilet is illegal, more so for the environment rather than immoralities. I feel sorry for the algae eater. It didn't deserve that. Also, flushing is the least humane way to euthanize as well. And I agree, males and females shouldn't be kept together unless 1) You are highly experienced with betta keeping and completely ready for backup plans, and/or 2) you are breeding.
Sorry for your betta loss.
I've been told corydoras cannot be exposed to aquarium salt too long as they're scale-less, so i see why it makes sense not to use aquarium salt with a betta without scales. At the same time, it may not be the same for both of them? I have no idea, I'm pretty curious about this now.
Why did you have to put the betta in the 1/2 gallon? IMO no betta belongs in less than 2.5 gallons (although 1 gallons can work). You can make it work with the female betta and guppies if she is not too stressed. If you want to keep her in the 1/2 gallon, please make sure you keep up with the water changes, or ammonia will go through the roof. Also, ADFs are fully aquatic... they don't live on land. And yes, that salesman probably was only
a salesman selling things. This is why research shall never hurt.