I apologize, I didn't mean to make it sound like you weren't trying to do right by him. I'm sure he's in MUCH better health than he was!
This is why I say what I do. Although equine vets do all go through the same schooling, they are not equal. Yours is not necessarily bad or any worse than mine (my vet would probably tell me to do the same thing with grazing and such, to be honest...) BUT many, many equine vets do not deal with miniature horses enough to realize just how different they are from full sized horses.
As a breed that is 'made' to not just live but THRIVE in poor conditions, miniature horses and shetlands can typically get along just fine on mediocre hay and a ration balancer or something similar. That is why I asked about the rescued mare in question's teeth. When a mini is given more protein, sugars, or fat than it's body needs, its body goes on overload and all kinds of health problems occure. They may just start out as being 'pudgy' but if you aren't careful they can become obese, and with obesity comes terrible things such as cushing disease, insulin resistance, and laminitis/founder.
All three are incurable, though for the first and last a horse CAN obtain remission from what I understand. The crazy thing is though, that even if a miniature horse is not overly fat looking or dead lame, it may have foundered and have coffin bone rotation. And it doesn't always take a lot to push them over into foundering, either. Many people associate founder with excessive feedings of grain all at one time, or a TON of grass after never having it before, but like with other health problems, there are varying degrees of founder. In one case the horse may be fine one hour then laying on the ground with swollen, bloody feet from founder and have to be put to sleep. In another the founder attack may only be enough to cause light sensitivity, but slowly corrode away at the hoof bones and walls... and even cause rotation.
I will use my mare as an example. She has been at optimal weight most of her life, but received poor hoof care and poor nutrition for most of it, before I got her. At 7 months old, just before coming to the farm where I worked, she was on pasture because it was assumed that she needed it while growing, and she was fed all stock Sweet Feed. Here is a picture of her a month ago, at perfect condition with decent feet.
At a glance would you think she has foundered? I wouldn't. In fact I never even knew she foundered until last year when I had a great barefoot trimmer look at her and tell me that her hooves had founder signs. I asked her previous owner and sure enough, she foundered as a weanling. Not severely, and not to the point of her not being usable not (she's a driving pony) but it was enough to deform her hooves slightly and make her MUCH more susceptible to foundering later. I have to be VERY careful about how much sugar she gets to keep her from foundering again if I want her to stay sound, so she is currently eating 2 flakes (8 lbs) of costal hay and one pound of alfalfa daily, plus probiotics for gut health. And she does great on it!
Her sire isn't so lucky though. He foundered last year after eating 4 lbs of sweet feed (broke into a bin) and it was severe enough that they ended up putting him down again due to very bad rotation in three feet.
All of that being said, I think a low sugar, all hay diet will best benefit this particular rescued mare at this time because we know nothing about whether or not she foundered in the past, how easy it would be for her to founder, or why she lost weight in the first place. Its easy to say she wasn't fed enough but teeth problems are such an issue in minis that I'd be very surprised if she wasn't in need of a float.
Grass, IF its nutritional value has been tested to be low, in moderation , can be fine for some miniatures but there is always the risk if it spiking in sugar ratios and the mini foundering. Sweet feed, I'd say its just horse candy and founder waiting to happen. It could easily be replaced (or even taken away completely) by a lower protein, lower sugar feed to eliminate the chance of founder completely.
As for the wormer, that one is a bit more flexible. I just don't really see a point in refilling a malnourished horse with toxins every week for a few weeks hoping
to kill the worms when you can do one dose and know what you're killing, then be done with it :)