The OLDER Empoleon is Better! The NEW one is really good as a TECH to the DP1
Backwards, unless everyone around you plays Banette.
30 to two places is more damage spread then 40 to one place. This can get more prizes at once. Heck, 40 doesn't even KO most pokemon anymore, only Shuppets.
70 plus a flip to possibly do 20 more damage to the next pokemon is good against Banette, yes. What about to Gardevoir? Not really, as the extra 20 or even 40 damage won't add up to a KO on Gallade (though it would to Garde non LV.X if you got both heads). Meanwhile Gardevoir can cheerfully three hit KO you, or Gallade easilly two hit KO, and their set up is flying by while yours lags slightly.
The answer would be 100 damage from New Empoleon's second attack. Plus the 30 damage already spread with New Empoleon's first attack, it KOs all 130 HP pokemon (Garchomp non LV.X, Gardevoir, Gallade, Blissey, etc.). The possiblity of damaging your bench isn't a problem with the Vaporeon, though that weakens your offense for a turn.
You're a bit confused on Prinplups. Old Prinplup could place 10 damage on everything, setting up even Gardevoir SW to get KOed by New Empoleon. Old Prinplup can attack with less energy. Overwave's a cool attack, but at that point, wouldn't you rather be looking for an Empoleon? It's an energy too high of a cost to be looked at, frankly. Regardless of how spready that attack might look to be, you might as well be attacking with Glaceon. We'll want DP Prinplup for same HP, but cheeper attacks.
Frankly, Piplup choice isn't too important. I do prefer the 0 energy cost 10 damage though to a 1 energy cost 15 average damage. :/
It's up to you if you want to try Mantine in here. It looks good, with everything important being water type.
I finally don't see as much of a desire to play Lake boundary. What is the extra damage going to effect? Magmortar? Already KOed by Empoleon (MD) and only 10 damage short on Glaceon. While Flame Body can heal Magmortar, it can't stop all of the damage from Empoleon MD's first attack. Blaziken? Already KOed by Empoleon. Magmortar? Same as Magmortar, but easier. Something in place should go there. If you're so concerned about Countering Crystal Beach... Maybe... Battle Frontier? Porygon likely won't be enough of a problem to warrent it, neither will Umbreon, nor Garchomp, nor even Delcatty, but who knows.
I wonder if Premier Ball might be a well welcomed friend in here. You want Glaceon LV.X for sure. Empoleon LV.X's ability to pick off Claydols is cool too, though not vital.