Don't think there's much difference between a properly used set of jackstands or an equally proper cradle... off season, unless the cradle comes apart the stands will be easier to store.
The top nut is just a locking nut for the one below. After the lower nut is properly tightened.. the top nut should be firmly snugged up against the first.
Use caution when tightening the keelbolts - try to make sure the hull is properly supported relative to the keel or you might to more harm than good.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"