Top line is usually the last to fill in. I've fattened a few starving souls over my life and it's always the last 100 lbs that seems the hardest. Most of the top line comes from muscle being built up. Exercise doing the most.
Being pregnant and starved I wouldn't worry too much about the topline. 8 wks isn't very long. It takes time and you shouldn't be doing much other than ground work with her in her condition.
Resist over feeding. Maybe start adding in a feed or vitamins designed for mares and foal. Slowly.
If you are overcome with the urge to put more feed to her make it something like hay pellets or beet pulp and have free choice hay or grass for her. The foal isn't going to seriously tax her system until later in the pregnancy.
Magic cell? I don't know what that is. Red Cell? Red cell has way too much iron in it for all but horses in serious hard work like top level eventing or race track. Better off with a broad multivitamin and mineral supplement, even good old ordinary calf manna. Dumor magic cell
? Think I found it. Generic Red Cell. Much better vitamin choices out there unless the vet specifically said she is iron anemic.
Tractor supply does carry mare and foal feeds and vitamins.