We use grass nuts as 'hard feed'. The calorific content is highter than hay, I would be careful of using them as a hay replacer, because apart from cost, ponies get fat on them, in much the same way they do on grass! Well ours do anyway.
Yes, yes and yes.
They are pure fibre (although short, so not as good as hay/lage/straw). IMO they are better than pony nuts as you know exactly what's in them (grass) and if you need to you can feed them year round BUT you need to be careful not to overfeed. As FestiveG said they have a high calorific content. We feed them year round to our oldie but not to the others, who do get some in winter.
You do need to check carefully though, one local supplier sold me alfalfa nuts as grass nuts once (I took them back).
I would say that they are the best 'hard feed' available for a native, if it needs supplementary feeding as grass is completely natural feed for any horse, unlike molasses & cereals, which make up pony nuts and mixes.
natives were/are bred to live half way up a mountain or moor and are bred hardy. adlib good hay and good water source will see them through if well wormed, they are designed to bulk up in summer to drop in winter which is why natives who live as intended rarely succumb to lami.
She came into winter underweight and has managed to put a fair bit on with alfa a chaff n conditioning mix. I recently switched to hacking mix as although still in need of weight she is not far off. Yard owner puts out minimal hay and we are not allowed to put more out sadly.
I would say that in your circumstances where the pony is underweight and not enough hay is being provided, then grass nuts are probably the best thing as reasonably calorific, fibre-based and close to the pony's natural diet.
One difference between straights such as grass nuts, and most commercial cubes and mixes, is that cubes and mixes are usually supplemented with vitamins and minerals. So if that concerns you, you might want to add a vit/min supplement to the diet (something like Equivite or Benevit). An alternative would be a balancer, which also contains probiotics/prebiotics which are said to help maintain healthy bacteria in the gut enabling the horse to make the most of the food it gets. Particularly useful if you feel the gut bacteria may have been damaged in some way, as a result of illness, stress, drug treatment etc.