What about key derivation. What are the chances that one could derive a seed by taking control of a public master key and something else? I've heard some concerns along the lines about key derivation which is why I just would avoid seed-based wallets altogether and would focus on the classic wallet.dat format but I haven't studied the details, I have just heard conflicting opinions.
I will leave this for smarter people than me
As far as 24 being too much of a hassle.. well I doubt you can trust your memory to memorize 12 words for the long term, so would need to type the seed somewhere, so if you are going to type 12 words you might as well type 24.
It depends on the way of storing the seed. The fact that many would just write the seed as it is somewhere (paper, file, whatever) it doesn't mean it's the best method. Some may memorize it like some sort of poetry, some may save it by making certain signs in a book or text (I've read that somewhere a few years ago, not making it up). The methods can vary and more words may indeed make it more hassle (while adding a favorite word would not need to even save it!)