If you write the seed phrase in a special book and are afraid other people can access it, you can use tricks like changing 1 or 2 positions of the words. That way, even if someone steals your phrase, they can't access your wallet. (You just need to change the position of the two words, no need to change too much to confuse yourself).
Yes, exactly. Duplicating the papers is also advisable so that when you lose one, you will have one as an alternative, and also using some words to trick it in such a way that you will be the only one to interpret it and to avoid stealing your coins, or just as you said, you should interchange the words in a way you can only understand.
Also, I think we should have a better storage device than paper because paper is very perishable, as you said. Small sized steel plates as mentioned by Crypt0Gore are a better solution than paper.
Using washers to save your seed phrase is also good, but it is a bit expensive. The full details and explanation were done by Fillippone in
this thread. I think it is secure and more easier to handle than paper, but the process is expensive.