As mentioned above, when an box of mac'n'cheese lists butter as part of the method for reconsituting it, you can omit it and still get quite edible results. Adding a bit of olive oil is a nice substitute for the butter, but not necessary.

As for boiling and then dehydrating pasta, I have not done it. I suspect that the only reason this would improve matters is that the boiled-dehydrated pasta might have a higher water content afterward than before, making it easier to reconstitute. It seems like overkill to me, but as I say, I've never done it. I should think that using a cozy and extending the 'cooking' time would be sufficient.