Yup, the crestiness is a sign of IR. That of itself is commonly a cause of laminitis(founder), but yes, the stress of being cooped up could be a cause too. Good on the farrier for recognising & alerting you to the 'sub clinical' signs of laminitis!
You shouldn't be cutting down on grain, but cutting it all together. The reason for feeding grain(altho there are generally healthier alternatives anyway) is for extra energy/weight. Sounds like your horse already has way too much of that! Your horse needs a 'low carb', plain diet, with *appropriate* nutritional suppliments, especially Magnesium. I'd stick to plain, pref. Native, grass hay & give a very small(like handful or 2) feed of copra meal or such, to mix supps in.
The reason I'd stay away from those feeds(& almost every other taylor made 'fast food') is I'd want NSC levels under 10%, I'd want low/no added Ca, and high Mg(not present in either of those at all). Have to say, I'm with the mob who don't think much of Purina, for one because they don't use fixed ingredients or give full analysis.
So, you do not want to feed grain at all. Due to it's high starch/sugars and difficulty to digest, it can be problematic, esp to already laminitis prone/IR horses. Beet pulp is high energy(& also high Ca), so I wouldn't be feeding much if any of that either. Whatever you decide, I'd look at feeding extra Mg & chromium too. Or use a supp such as Heiro, Remission or GrazeEzy