It's just an inherent problem in the design of the suit. If you want to make 3 sizes that fit everybody, you're gonna have a suit that is the perfect size for 3 people, and varying levels of compromise for the other 97% of people. The problem in all of that excess designed for someone bigger has to go somewhere. The outer skin does a great job at streamlining the profile that sees drag, but does nothing for all of those potential air traps that comes with the excess material inherent to the trash bag suit design.
You're just going to have to be extremely judicious in flopping around to move all of that excess air to a position where it's able to be vented. It's just a fact that they are really good at trapping air, which equates to really poor performance if the suit is in any way too large, which they all are, unless you're one of those 3 people that perfectly fit in the thing. Unfortunately, there's really no way to minimize the amount of excess material in the suit. You could try and pay someone to cut out the excess, but then you're playing with fire when it comes to potential leaks.
What was it about a telescopic torso that you didn't like? Did you get to try any of the suit styles before your purchase? I find a lot of people buy Fusions because dive shops use them for rental suits. Instead of having to stock a wide variety of suits that fit, they only have to stock a couple sizes that mostly don't fit, but most people renting drysuits don't really know any better. A lot of those people then end up buying them when they finally decide to bit the bullet because it's the only thing they're familiar with. They don't realize the low price is a reflection of a number of compromises with the suit design.
Just a few counterpoints:
1. I agree that the core of the fusion suits are cut very big....and I dive one that is a size larger than I currently need (I bought a 2xl/3xl when I weighed 240lbs and now weigh @ 188lbs). I took the skin off mine to hunt for a leak before shipping it off for warranty replacement and the core is cut huge....
...However I don't find my suit traps air at all and I have not experienced any negative impact on my buoyancy management.
2. I find that the skin keeping the core streamlined slows the movement of the air bubble in the suit allowing me to be able to feel the bubble move slowly across my shoulders or down my back and legs. This allows me time to react and change body position to manage my spatial orientation and comfort in the water.
3. My wife has tried a tri-laminate suite and now has a custom made neoprene drysuit...the air bubble in her suit moves quite fast making the learning curve to control it steeper.
I believe the flapping around the OP mentioned is part of the normal learning curve of figuring out how to dive a drysuit. I don't believe this would initially be any different if she had a different style suit...but I firmly believe the way the Fusion is designed that it should help cut the time before she is able to dive her drysuit proficiently.
4. Fusion drysuits are far from "low price"...They do offer a nice set of features at a competitive price. I think they are well designed for folk who are not sure what kind of fit they want/need. Fusion suits can be one's first and last suit or they could be a stepping stone to figuring out what one wants in their next suit. The great thing about the Fusion suits is that one does not need to be uncomfortably snug or have lots of extra room and air to manage if their off the rack suit does not fit properly.
5. The fusion suits are designed to be adaptable to changing body morphology. Had I purchased a different suit back when I weighed 240lbs, I would be suffering on every dive now that my body size has changed. While I could use a smaller suit, my Fusion suit still works great...and will adapt to my body should my morphology shift in the other direction.
6. There is no one best suit on the market. What is ideal for one diver can be the pits for the next.
If I could go back in time to repurchase my very 1st diving drysuit, I would buy a Fusion again. Would I buy one for my second diving drysuit? Depends on how the suits I try on fit when that time comes.
-Z