It depends on the type/brand of conditioner, and how much water is the Betta tank can hold.
A betta fish would be much more suited to a 1 gallon tank than a 'variety'. You shouldn't keep much more than one fish per gallon anyway.
A betta fish can last 2 - 5 years if kept in a tank with proper conditions.
it will be sick but if it continues then your betta will die.
atleast 2 times a day .. once at morning and once at night
Bettas are dumb as rocks. If you put another betta, any other anabantid like a gourami, or any long-finned fish in with a betta, the betta will fight it because he will think it's another betta. If you put a short-finned fish in with your betta, they'll live together peacefully.
For the best health and lifespan of a betta, you should consider something that is at least 2.5 to 5 gallons and can have a small heater (bettas need a water temp of 78F to 80F), and a low flowing filter on it whether that be a small internal filter, or a small HOB (Hang On Back).
I guess but I don't think the fish would enjoy being in there that much.
A pinch of fish food once a day
the betta will eat as much as you give him pretty much. his stomach is the size of his eyeball. if you overfeed your fish it will get sick and die. feed it once or twice a day. as much as he can consume in 3 mins
Pretty much anything including other fish
Neomycin sulfate. Not sure how much or where to get it.
You can, but make sure you catch your betta's attention if you add more food, if the fish sees the food then there is a much better chance it will be eaten. Also, make sure that the food is a small enough size that the betta can put it in it's mouth. Finally, you should remove any uneaten food that the betta doesn't eat within 5 minutes or so, as this will contaminate the water over time.