Hello, Susan!
You and your family are going to truly love the Treehouse Villas at Disney's Saratoga Springs Resort & Spa!
Regarding your grandmother, if she moves well, I think you will have no problem. That being said, let me tell you to consider the following: each Villa is, obviously, raised quite a way up from the ground level. If your grandmother is fine on a full flight of steps, this will not be a problem for her. The stairs are sturdy, with modern hand-railings to provide support as well.
The walk to your bus stop shouldn't be too difficult, either. In comparison to the walk we've had from our room to a bus stop at other Walt Disney World resort properties, the walk to the bus stops at the Treehouse Villas seem quite leisurely and level. You will, however, need to take that quick bus ride to Saratoga Springs Resort and Spa (it's literally a three-minute ride from TV to the first bus stop at SS) in order to go anywhere else. Get off at that first SS stop and wait for the park bus of your choice there. Because the Treehouse Villas are built on wetlands, this helps prevent an inordinate amount of bus traffic there, and, quite honestly, it keeps the bus noise level way down, too!
I hope that this answers your question regarding your upcoming stay at the Treehouse Villas at Disney's Saratoga Springs Resort & Spa! Should you have any additional questions, please do not hesitate to send them my way! I'd love to help!