Of course it effects all of those things. Its a compromise. Any of those methods in moderation wont have an adverse effect. its a matter of what you want. In my early days, I had trucks moderately lowered with these methods, and they were fully functional trucks as well as being driven daily.
A truck or van lowered with engineered lowering components is going to have compromised load/towing capacity unless provisions are made.

 

the fronts on my 2002 1500 seem easy.
it looks to be the exact same thing as the chevy c1500 trucks around the 96 year model

i might do the axle flip but its an 8" drop alone and would bee a c notch
i need to see if i can fit airbags for towing with the wheels i have in the rear

 

These vans are designed to haul a s-ton of weight. And when you do, the van levels itself out. Same with pickups.
The firmness of the rear suspension along with the 80psi in the rear tires can be felt while driving. If you haul less than 1k lbs, it couldnt hurt to soften it up some. I myself, am not a fan of the stinkbug look either. For mine, id be fine leaving the front the way it is, and just dropping the rear 2 or 3 inches by de-arching the springs.

Taking a few leafs out sounds possible, for a softer ride, but then hang on a minute, were talking about the only thing firmly attaching the drivetrain to the vehicle, having that sort of rear end twerking cant be good. Only if it was Miley Cyrus.

Nobody makes drop shackles.
The tires are the same as the front, and on the front are spec'd to 50, with that heavy engine. I say go for less psi, and a dearch. Skip the whole slammed look, try pulling into a gas station with a steep ramp with that garbage, or hitting a pothole with paper thin tires.

 

We have a 2005 2500 express van, flip rearend and airbags, 2005 the frame is c notched already, front used 1993 stock 1ton duelly coils. running 295/40/20 front and 305/40/22 rear on 10" wheels.


Now the problem I have always had is bump steer, it is real bad. I wash someone knows how to fix this.