Firstly, not knowing what your grazing's like or the hay, can't comment on whether it's not enough or too much, but if she's been on this diet for some time & still underweight, I definitely wouldn't say it's too much. Horses need a minimum of 1.5-2% bwt daily of forage. For weight gain, 2.5-3% or more. You can also feed ingredients such as beet pulp or such, which is a high energy, high fibre but low starch alternative/additive to the chaff.
Regarding her scouring, sounds possibly due to hind gut acidosis, so likely sugary/starchy feeds at fault. 'Good quality' improved grass hay such as rye for eg can be a problem there. Fresh grass will be the same - depends what it is, etc as to whether it could be problematic. Too much alfalfa, clover or such can also be a prob. Oaten chaff/hay can be quite high NSC & can also include oats, so be aware of that. You can always soak the hay/chaff for a few hours & drain it to leach out some excess sugar.
Re her weight, has she been wormed effectively, teeth done, checked/treated for ulcers, particularly after the scouring? Probiotics may be a good move too, especially after gut upsets such as that or drugs. Re supplements, you need to find a good quality supp that fits your horse's specific diet. One good way of doing this is to get onto feedxl.com