Hi. The most difficult thing about stopping feeding nuggets is getting ourselves out of the mind set that they need them. I feed them once a week during the summer months and twice a week during winter, approx 20g per pig each feed. I regard them as junk food for piggies with the benefit of added vitamins and minerals, I think of them as a treat not a dietary staple.
I feed the rosewood naturals you mention. They are very different to most other pellets, bigger and harder and you may find your piggies are not keen at first, some of mine ignored them to begin with but all enjoy them now. I like them because they are both grain and soya free.
vitamin D is found in hay, it is absorbed from th sun while drying. This does not happen in other kiln died products such as rreadigrass. It does deteriorate with time tho so new season hay will contain more than hay that has been stored longer. In the UK hay is cut annually usually in June..
Calcium absorption and metabolism is very complex. Unlike us, guinea pigs absorb all the calcium in their diets and excrete any excess, where as we just take up what we need. The calcium phosphorus ratio plays a part as does vitamin D. Also oxalate is an important factor as it is a bladder irritant and in excess will cause bladder issues. Personally I avoid feeding high oxalate veg such as spinach but don't worry about high calcium veg such as brocolli and feed it daily. Calcium binds with oxalate and removes it safely from the body so the danger is that a low calcium high oxalate diet can lead to more problems than a high calcium diet. Soya is also very high in oxalate but in a form that is not absorbed by humans, however I don't know if this applies to piggies too but is worth baring in mind.
Fresh grass is the most beneficial food guinea pigs can eat because it is their natural diet, balanced for their nutritional needs and what they spent millennia evolving to eat. I believe the more grass based a diet can be the better it is for every aspect of guinea pig health.
All that said, Dot may be pre disposed to bladder problems so all you can do is your best, which is what you are doing. I have had pigs with bladder stones before which never returned once I stopped feeding nuggets (and grain such as oats) which is now why I severely limit them in the first place.