Anna - The flushable liners would be ok for the NB tar poo, and for later on, when going out they're great. But hardly used them here for the first 4 months.
Prewashing just needs to be done in water. It's mainly to remove any residues from manufacturing, and to build up absorbency depending on the fabric. So bamboo and hemp need up to 7 prewashes, but microfibre and PUL just one. There's more detailed instructions and info on sites like nurture nappies.
I currently don't use detergent to wash DS's nappies and if I do I only use a little bit of Eco Store powder. Hang them in the sun and no smells or stains, except for one BumG booster that's been stained by panadol and is never coming out apparently

WOHB: berryplush