You're right about the three hands. I'm pretty sure that a couple of us could manage, it's just an O'Day 23 and pretty straightforward. If the lazy jacks weren't secured anyway the sail pack sides would just hang listlessly out of the way of the gooseneck. I usually attach the topping lift and then go to work on that pin that holds the boom to the mast. You're making me think - taking the mainsail off is smart but I might not take the sail pack off. That raises the further question of running the lazy jacks up through the blocks up above the spreaders. It's a keel stepped mast and requires a crane to raise it. Hmmm, messenger lines while it still lying in the cradles?? Just when I think I know how my boat works something new comes along
With the sail off of the boom, attaching it should be a 1 person job. As you suggest, attach the topping lift to support the weight, then go forward and fiddle with the goose neck. That's what I did on my Tanzer 22, the Sabre 30, and do on my Sabre 362. Take your time, it can be done.
Yes, run some messenger lines for the Lazyjacks while the mast is down. All you need is some cheap line, mason's line would work, to pull the lazy jacks back up.
Bending on the mainsail can also be a one person job. Two people make it a little quicker, but one can do it. Attach the foot to the boom, then raise the main. Have all the parts and pieces in your pocket. The only tricky part is putting in the track stopper to keep the sail from falling out of the mast groove. Raise the last slug past the opening, then step on the halyard tail to keep the sail from dropping while you put the stopper in. Easy Peasy.
The only job that is challenging for one person is bending on the jib in a furler. The problem is feeding the luff into the groove. While there are feeders you can buy, it almost always requires some one to help feed the sail into the groove.