May be it not time to say it but i want to give one example .
You ask if you should give up i would say no because i wish you can safe one more betta from the store and hopefully give it a live. And i believe that you really care because i read every post you put, medications bought, water changes, your updates etc...i believe that you really care and should try again and we can figure out or try to figure out what was wrong. All disinfection's, environmental issues so on we can try to figure out....and i think give it another chance.
I don't know but i really want to write something that really bothers me, may be you feel better ...that you did everything you could
I have a coworker that came to me today. She is a lot of older than me. I have 2 betta on my work table. She came to me and told me that her son in low bought a bettea. Now it dying. It don't eat and just hanging on the top for a week already. When i asked her to bring me a betta to see what wrong and treat him she say:''no no it just a fish i don't know why they bought it, they have more stuff to take care of . They have child''. That exactly what she said . Then she added :'' it i hope will die soon''. And fish still alive. So that is example people don't care .....i hate people sometimes. That person don't understand when i say that i feel bad for dog or fish or....Those people should never have any kind of animals.