His mother was sitting on the board of IBM when they went looking for a replacement for CPM. Gates didn't develope DOS,but bought it from the developer. Nothing wrong with that,and he is responsible for further developement of it.
Nope. Jack Sams (I believe), of IBM, had approached Microsoft for CPM, thinking that Microsoft could produce both BASIC (and a few other languages) and CPM in one nice contract. Microsoft didn't have the license to resell CPM, only to produce the SoftCard, which ran CPM programs on an Apple computer. Once IBM realized this, they went to Digital research, who basically blew them off. Gates, in a gamble of the century, told IBM that MS could produce the needed OS. They did, and the rest is history. Gate's mom played NO known role in this. Microsoft had already made the required name for itself as a leading language producer, and, to IBM, the producer of the SoftCard, a CPM host. IBM wanted languages and CPM. They thought they could get both from Microsoft.
There was never a replacement to CPM as CPM was never produced for the IBM, and IBM was already doing business with Microsoft when they needed an OS because Digital Research didn't want to play.