RE: Minimum Power Requirement
The wing planform looks fine. But with that much sweep you need some twist in the wing or something that takes the place of the usual twist. Such as making the tip airfoils lifting but cambered upward instead of down so that the tips can provide the stability needed. If the wing is truly flat with no twist and uses the same airfoil from root to tip then bad things are going to happen since there is no provision at all for stabilizing the model in pitch.
Which brings up the idea of equivalent wing area. Or more accurately determining the localized lift coefficients of each wing segment from root to tip and using calculus to determin what the overall wing lift coefficient will be. And I can assure you that it will not be equaly to 1.0. The center section area may be lifting at 1 or even more depending on the chosen airfoil. However there's no way the tips can lift the same amount if the model is flying in a stable manner. Be it from twist (which you aren't using according to the wing guy) a graduated shift to a downward lifting airfoil at the tip or the use of strongly reflexed elevons the tips will be much reduced for lift coefficient and the overall average lift coefficient will be more like .6 to .7 or maybe as much as .8 if the center section is using a strong lifting section with a max Cl of up around 1.5.
All of which is going to raise the minimum airspeed.
You need to go talk to the wing guy to find out more about this miracle wing or to understand where the communication breakdown has taken place. Something is just not right here.
And what is wrong with a takeoff dolly that you roll away with and then lift out of it to leave it on the ground while the model flys up? It's either that or some form of launch rail. Also since you're using an engine you need to be careful how you get the model up to speed. Sharp accelerative loads like javelin throws or heavy bungee launch rails (I'm assuming that a huge slingshot like deal is what your water balloon launcher is) can accelerate the model so fast that the engine starves for fuel and quits. The bungee powered balloon launcher is the way to go but to give the model room to get up to speed before it looses support I'd suggest a bungee powered launcher rail that is about 10 feet long. And plan on a speed at the end of the rail to be up around 30 to 35 mph just so you're safely flying when it leaves the support of the rails.