If I may address your concerns, valid as they, in your mind, may be.......
#1 I have "lost" two Windows Mail accounts due to changing ISP's. I had no problem regaining access to my Stardock account. They, and you, have ways of proving who you are.
#2 That would be a bad deal, but mainly from a junk/unwanted mail point of view.( many, many people post their email addy's in the forums, God only knows why)but that blame rests squarely upon Yahoo. That fix is rather simple. If you have such concerns, don't use a Yahoo Mail address. Anywhere, anytime, for anything. Period.
Note that PayPal is one of the most trusted sites on the planet and SD 'uses the same format, and in my mind is just as secure.