Diapers are for incontinence and girls in heat. Loki has nighttime incontinence (now he's on medication for it) but when we were still figuring things out my vet told me not to use belly bands. Urine can scald the skin. As someone who has had to give many baths to a wet dog, I would not choose a diaper. (The fur soaks it all up!! Even if they are trimmed short!) I'd rather he pee on my nice white carpet than on himself. Easier to whip out the Bissell than give the dog a bath.....
Some people use them at other people's houses. I found that Loki had less accidents elsewhere when he was a puppy, because we watched him more and took him out more. As far as letting the puppy out - just do it! If she pees on the floor she pees on the floor. Just supervise her and keep the trips short. Just try to make sure she's empty first
We have an open floor plan house, so Loki was always either tethered, in his crate, or running free and we just watched him. Trust me, spend $60 on a mini Bissell (mine is called a Quicksteamer) and enjoy your pups.