Yes, you can sit on the closed lid.
It's not available as a spare part.
Yes, it comes with all parts needed. My grandson installed it easily.
Call the manufacturer and they will send you mounting hardware replacement free of charge.
Both the lid and seat remain open or up. You can open them together or desperately. And they both slow close when lowering. If your seat is not staying up you most likely didn't install properly.
Multiple options to choose from in the store. Great quality, easy to put on our toilet.
Open the two tabs on the back of the seat and pull the entire seat towards you. If it doesn't move you may need to unscrew the two attachment screws a little. You don't have to remove them.
Easy to clean and very quick to do.
Yes, it will fit any elongated toilet.