Pellets start out as hay (obviously), and while their DM may be higher at the end simply as a function of being compressed and pelletized which involves a die and some heating due to pressure, it’s not significant.
Equi-Analytical puts alfalfa pellets at an average of 91% DM, with a range of 89-92. It puts legume hay at 90% average, with a range of 89-92. That’s insignificant.
Protein is average/low/high higher with legume hay than alf pellets but when the lowest of that is 17%…
ADF and NDF average 30 and 38% for legume hay, 35 and 47% for pellets, meaning there’s more alfalfa that’s more mature, being used for pelleting. Those are functions of digestibility and palatability, and while higher (which isn’t always good), the pelleting process increases digestibility of something, so that’s probably not an issue either
DE of pellets is listed as 1024 average, hay as 1101, also insignificant.
Quality is about a lot more. Many companies use higher quality hay to start, some more local mills use “leftover” that wasn’t good enough to sell as horse hay.
So no, it’s not a 1:1.5-2 ratio, it’s pretty much 1:1, all else equal.
Some do, lots don’t. Many don’t list anything other than “alfalfa”. Some list protein, fat, fiber. Some list protein.
Nutrient value is always on a dry matter (dry, as in not soaked, not the actual tested DM) basis, as water has no nutrients or calories. IF pellets were, say, 1.5x as potent as hay, in most ways, then theoretically you could feed 67% of pellets to equal 1lb of hay, Soaking would have no impact on calories or nutrients, only volume.