I agree on the fertiliser question - it would be rare for it not to be risky for horses re laminitis risk to use artificial fertiliser, certainly without seeing the effect of the other measures first. Horses are designed to eat 'poor' quality grasses in cattlemens terms, not a green sward.
Not trying to put you off the rest of the measures am concerned about the fertilising. Grazing hard can make grass grow back short sweet and stressed esp if liming etc brings the ph up, so sugars will be higher anyway.