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.
1984 Fast/Nicholson 345 "FastForward"
".. 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)