If you had a male and female in the same tank, there's a 99% chance she's pregnant. You'll start to see her gravid spot get bigger, and her belly will get larger. Eventually her stomach will start to get a boxy look to it, and that's when she's about to give birth.
Yes, it's the dark patch on their stomach. The best way to raise the fry is to have them in the tank with a ton of hiding spaces. The strongest, healthiest fry will survive and you'll have a reasonable number. By keeping them in the tank they also have access to all the microorganisms that grow in there to consume as food. You'll know the female has given birth when she is suddenly not so big!
We're gonna need more info to answer that. Generally, you could do like 3 water changes a week?
But I can't give you a good answer if you haven't even set up the tub yet, etc. A tub with bunches of floating plants could go like 6 days, no water change. A tiny plastic bin with nothing would need water changes every day probably.