I've had to replace the coolant in my Yukons a number of times for various jobs including cam swaps, new radiator, coolant tees, etc. I never had any problems with air in the system or anything else. Modern coolants are remarkable in their protective properties, and I've never seen any sludge or corrosion debris in any of my cars with modern cooling systems, even after hundreds of thousands of miles. I say put new 50/50 Dexcool in it, and send it. No need to flush. I'd be worried about residual flushing chemicals remaining in the system.