Sissy's singing was so good,

Sissy did so much more than just recite those songs. She adapted them in her own original renditions so the songs came out with her own indelible stamp and what we hear is Sissy Spacek doing incredibly wonderful justice to the feeling and true meaning of those songs. I've never seen such an honest and moving re-working of songs as Sissy Spacek achieves here. Her heart is revealed every time she opens her mouth to sing and when she does open her mouth to sing, it's always out of pure joy and the sheer enjoyment of self expression. Even when she burns out from exhaustion on the road, Sissy's capable of singing the pain out, which takes a degree of honesty and artistry to successfully convey. However, she does the task admirably. It's hard to believe she's not a professional singer, considering the charm with which I enjoy humming in my mind Sissy's renditions of these songs rather than the original sources. Her softness with which she translates the songs is absolutely delightful and my memory contains both Sissy's renditions of Loretta's songs as well as Patsy's.
These 2 actressess get straight A's in my classroom.

She was absolutely fantastic in this film (as usual).

She did a great job in this movie.

Sissy Spacek sounds more like Loretta Lynn than Loretta Lynn does.

Does anyone know why the version of the title song that Sissy sings, has the second verse omitted vs. the Loretta Lynn version?

Yep - she knocked it outta the park.