I've never done a lapstraker, but I'm not seeing a problem. If there was to be any unevenness, I'd think it might occur at some of the edges of your "planks", but that's not to say I'd anticipate any problems. I'm thinkin' you're going to have one shiny boat.
Start doing a few pushups now
if you're going to be working on your back. Standing on a stepladder and holding up a big grinder in front of you for hours is tiring. Holding it straight up is bound to be even more fun. I could see myself skimping a little below the waterline....heck, the fish don't care.
If you could flip the boat....woo hoo.
Hey....I just reread your post, and noticed that you're starting with the Finesse-it. If your gel coat is currently close to fresh out of the mold shiny, that's a good way to go. If it's getting a little dull and oxidized, definitely do the compounding. I used the 3M on mine, and it was fine, but I think next time I'm going with the Presta. I understand that you're concerned about burning it. Pity if that concern kept you from getting gloss after a lot of work. You could always practice on the keel.