So Mr. Valve just gets his brother to make a 1-line cosmetic change to to Dosbox, gets a copy of that with the GPL attached, and then has a licence for a different program that he can redistribute with all his steamy software.

No, it doesn't work that way. You need a license from every copyright holder to copy the software. If any one of them has terminated your license, you can't copy the software anymore; it doesn't matter what else you do.