A seat back would render the swim up deck on the back of this kayak useless. My kiddos have zero problems with no seat back. They prefer it over the kayaks that do have them. A simple boat seat cushion from Walmart can be placed to pad the seat and will also float if the child (by some off chance) flips the boat. Mine has only done it once and it took some serious goofing off in some rapids to accomplish the boat flip
I ordered the pink one and it did, it is a two piece paddle that comes apart in the middle with a "push button" to store.
My 7 year old has taken it down a river with small Rapids. Most of it depends on the age of children that are on it and their comfort level on the river. Adults have no balance on it.
Scud plugs are super easy to buy here on Amazon. Just search ;)