From the photos provided, I would say it is *possible*that she is a minimal dwarf-the roach back, distended belly, and what appear to be pretty cow-hocked hind legs would, to me, indicate a mild "short-upper-limb"(my own term)form-but, it is really hard to say, one way or the other, from the photos.
Kudos to you for loving and caring so for her-that's what counts-that you love and are enjoying her! I agree that a regular worming regime is important; depending on her previous deworming history, the fact that she had recently been dewormed doesn't necessarily mean she is worm-free. You may well need to do so again at 30 day intervals, a couple of times,with appropriate products(do not use anything containing moxidectin), AND at accurate dosing-people often underdose, and that only helps the worms develop better resistance to the dewormer(s)! Then, continue with a suitable regime(around 60 day intervals, alternating 2-3 chemical 'families', at proper doses.0
Given her age,I would agree that Purina 100 probably wouldn't provide the nutritional level she needs;you could go to Purina 200, but if it were me, I would consider a 'complete',junior feed(as Magic suggested), such as Equine Junior(along with reasonable amounts of a decent grass hay)-but I would suggest caution in amounts of ANY concentrates(meaning grains, and feeds formulated with grains). I have NEVER fed as much as 2 cups twice daily of ANY concentrate to ANY of my minis, with the possible exception of large(34")mares in full lactation. It should be remembered that horses evolved as grazers, and need the long stem fiber of natural grazing, or of hay, as a substitute for grazing-their relatively inefficient, and somewhat fragile, digestive systems, were NOT designed, through evolution, to utilize relatively large amounts of concentrates well. My advice would be-to be cautious, have an accurate read on her weight, study manufacturer's and researcher's opinions/recommendations on amounts to feed, with specific reference to Miniatures. The distended belly*can*be a sign of "regular"sized intestines in a smaller-than-usual body,IS one of the signs of dwarfism, can have little to do with what the horse is/isn't eating-so might not be a sign of overfeeding of hay. It does appear to me that she is a bit-not hugely-overweight, overall-it might be that a more nutritionally suitable concentrate, but not necessarily more OF a concentrate, is called for.
As for her foal photo-though it's not too clear, I would say you are correct in your assessment that even then, she had a bit of a pot belly. It is common for minimal dwarves to not be very evident when young, becoming more evident as they get older. Even if your little mare IS a minimally-expressed dwarf, she looks healthy and well-loved---and that's what's ultimately most important!! Best of luck with her!