Leveling the sagging side of the home
Sure you can fix this, but I wouldn’t opt for the “shelf” idea. 

Typically, in a post and beam foundation, all loads are transferred directly to the soil. Installing a “shelf” will put thrust on the foundation wall. Obviously the foundation is marginal and I doubt the wall is designed for any horizontal thrust. 

You’ll be pouring several “pad” footings, so why not pour one near the foundation wall and eliminate the “shelf”?

Btw, Is the roof really sagging, or is that gutter installed in such a manner that it appears to be sagging. If you look close, you’ll see there’s about 1 1/2” of fascia board exposed under the gutter on the left side of the house, but about 3” exposed on the right side. (Although, that roof section on the far right looks like its sagging.) 

Also, if we’re sagging very much where you have the arrow pointed, that window would be cracked/broken. 

You’re lucky that it’s a one story house. The loads are easy to follow from roof, to walls, to floor, to footing. 

If you’re located in a high seismic zone or high wind area, you’ll need extra holddowns so I’d contact a structural engineer or architect for help. Good luck.