aww that is terrible, dont get rid of your old cat so quickley! i dont understand how people can say they will dump there cats so quickley. It could take awhile to figure what is wrong with a litterbox problem.
your old cat urinated there because she smelled the fresh pee. no matter how much you clean it, the cat can still smell it and will cover it up with their own smell. that is just what cats do...try different products and it might go away.
your new cat may have peed there because he was new to the home and got confused and scared. the best thing to do it to keep the cat in a small room like a bathroom with a litterbox and food and water for a week or so to learn about the litterbox and to also recover from what you had him go through.
your newly declawed cat is in an incredible amount of pain. they dont show it but he just had the tip of his little paws ripped off, he is going to need plenty of time to recover. Because you declawed your new cat he could grow up with all kinds of litterbox problems. cats are meant to have claws and taking them away means they cannot defend themselves, they cannot balance the same, they cannot EVER go outside, they cannot stretch and strengthen and develop the muscles in their arms. It is very traumatic and unnecessary to have put your cat through. be very patient. some cats won't use the litterbox after being declawed because it is too painful on their paws.
i guess it is better that a cat have a home and be declawed then to be left behind in a shelter or out in the wild. it is illegal in many other countries, I wish it would be everywhere.