Coop, no matter how narrow or how wide the piece, I always start in the middle. Starting in the middle you lay-down the finish thicker and work it out to the edges. Should you start with an edge and get a little thick, ya get runs. I like this method especially, so your not working over your freshly laid finish for clothing to touch or what have you.
Your four wet edge concern, the way I reconcile it, I have only 2 wet edges. One is where you start, the other is the edge you're continually working towards you. Make sense?
The wet edge is key for the finish to blend in and become one. Have you ever had to pee so bad while rolling paint on a wall so you stopped in the middle of the wall. Came back and started to roll and now you have a ridge of dry paint that shows where you left off.
The key on any large surfaces is to move fast and keep it wet.
I didn't have that problem, at least. All of my stock seems fairly stable so far. I think the jigsaw just couldn't do it with the blades I've got. Your bench was the one from the ash log you cut up and air dried, wasn't it? All of my stock is kiln dried from the dealer, and may have been around for a while since I guess they move a lot more 8 or 10 foot stock rather than 12.