Your mare looks to me like a poorly girl. If only I could I'd send you some of the fat off my Irish Draught X Connemara who needs to lose some weight. IDs as a breed are good doers and put on weight easily if treated well.
I feed my mare grass, a pasture mix with vitamins, plus the occasional apple, carrot, parsnip, swede. To get medecine down I give her some sugar beet
SHe also gets a salt supplement as many pasture needs added magnesium
Mine also gets a molasses lick in the winter months. She is fed oat straw or alfalfa if she gets too fat.
If your horse were mine I'd check: Teeth, & worms, and I'd call the vet - which you have done.
I'd buy a tape to measure her waist and weight on a daily basis and I'd record everything I fed her
With a horse that thin, I'd worry about saddle sores
But the real worry is giving her too much too quickly - she'll be at risk of both colic and laminitis. She needs to build up slowly. Maybe fed three or four times a time - little and often.
My girl has a blanket when the temperature drops below 5degc.
She has a raincoat if the rain looks to be heavy
Have you tried sending an email with photo to one of the better known animal feed compounders. They'll come back with some ideas, If the Aussies won't , then send to the Brits - they will reply I am sure.
If she is a crib biter then some horses inherit the habit but some develop it thru stress. If she has been starved or neglected then that is stressful. Wood splints can lodge in the throat but you would see signs of a blockage.
I'd watch her eat too. Does she want to eat? I'd walk her in a halter to grass
And make sure she ate.
Well done, that horse needs some Tender Loving Care. I wish you well. But look out for behavioural problems. A horse is motivated by fear, food and sex - your horse may have had too much fear, too little food and no sex.
She'll need careful handling and your risk now is colic - perhaps followed by laminitis. Poor girl, she doesn't need either of those problems.
Maybe I ought to show my mare a photo of yours, then she'll know how well off she is now. Trouble is, she already knows I am a sucker.