I'm assuming the details of these "secret investigations and private communications" proving the drivers are different are not available for anyone else to see.Believe what you want, for myself I am pretty sure (because of my investigations and private communications) that they are different drivers made on different specs, but please check other sources as well. Moreover, you can buy a lot of variations of these drivers (magnetic circuit, impedance, diaphragm, etc) on Ali and will cost and measure and sound quite different. Granted, all of these do share some common elements.
If you believe they are the same drivers, then go ahead, buy the D5200 and mod them as you wish.
Changing the cables helps indeed, but of course you can't change the wire from the voice coil .
Not for the D7200 - its own modded pad is the best. Or look for shallow leather pads with bigger inner opening, e.g. these (even tough a bit thick, but cheap). Or some Vesper Audio pads, like this or this or this. But as I said, the headroom is small. There are some drivers on Ali that are cheap and a big change if you are bothered by the 3K ringing - but this is a big change and not easily done, and likely not reversible (I didn't do it, but @Maxx134 did).
Anyway all I'm saying is the specs are almost identical for all 3, it takes allot of r&d to develop a driver from scratch, most likely they are using the same driver. I have no proof to verify this but neither do you. Its all speculation because no one is going to destroy the drivers to verify this. At least i can point to the specs and the drastic changes to the sound from the mods and not just going by Denon marketing hopes and dreams. If you know something we don't, please feel free to share.
I have bought and tested many drivers from AliExpress and find that most of them are hot garbage, amongst the drivers I've tested up to the $80 price point anyway.
No you're not going to get a close to d7200 driver off Ali for $40.