If this was true, that it is literally impossible for the passengers to have made those cell phone calls(or airphone calls that someone mistakenly described as cell phone calls), and basically the entire planet was told they made these calls, wouldn't it be inevitable that countless qualified people would point this out

There were communications experts who pointed this out, but they only did so on an individual basis. Hundreds of people individually pointing out the impossibility of cell phone calls from altitude has very little collective effect. Their voices get buried under the mountain of people uncritically repeating the official story. Out of those hundreds of communications experts, they might on average make a few comments online (or to people in real life) pointing out the difficultys involved, before they cease to speak anymore on the matter. Besides, without actual experiments to prove this matter definitively, they could not be absolutely sure about how impossible it was.

Nobody thought to look into it until AK Dewdney(whoever that is) did some "experiments".

If you are curious about his experiments with cell phone calls from various types of aircraft, you can read more in this link. Project Achilles: Low Altitude Cellphone Experiments.

How do you recruit numerous voice actors that can flawlessly imitate someone else, and have them successfully talk to those people's closest relatives and associates, and fool them all?

You don't. You tap the phones of the future hijacking victims, record samples of their voices, and then use voice morphing technology to replicate their voices. This technology was successfully demonstrated in 1999, enabling its user to impersonate Colin Powell.

What about the actual cockpit radio communications?

The voices of the hijackers communicating by radio could have been pre-recorded, or made by live actors. The radio messages themselves could have been transmitted by either the hijacked planes, or the drone aircraft they were swapped with.

What happened to the passengers themselves? How many people had to be involved with kidnapping and disposing of them?

It is not clear whether all of the passengers on the planes were innocent victims. There were Mossad agents onboard Flight 11, for instance. At any rate, the passengers did not meet their fates by crashing into the Twin Towers or the Pentagon: That was the work of drone aircraft. The Boeing 767 aircraft which they departed on were swapped in mid-air by KC 767 tankers (prototypes). Drone swap was the same method proposed in Operation Northwoods. If the maneuver is pulled off precisely, it can even fool the radars of air traffic control.

It just gets worse and worse the more you logically delve into a lot of these more elaborate 9/11 conspiracy theories

9/11 conspiracy theorys may sound implausible on their face. But the official story of 9/11 is downright impossible in many aspects. And as Arthur Conan Doyle said: ''When you eliminate the impossible, whatever remains, however improbable, must be the truth.''