1) Sounds like you're describing a Yo-Zuri or Braid Products style Marauder (aka Bonita Bandit). They're commonly used for offshore fishing. I don't think double hooks would do much more for surf casting that trebbles wouldn't. They are also usually much longer shanked than trebles so that could effect action and fouling. I know guys have tried single hooks with some success.
2) Already seen that idea in print, though using EZ Body tubing or similar braided mono tubing, then painted with Sharpes or something. Don't know how you're going to stretch a tube over a large Danny size wood plug anyway.
3) Broken back/jointed lurs are meant to simulate more of a fish's wiggle as it swims, especially the tail. Not much wiggle in a fish's belly (unless it's a Santa Claus fish
).