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In terms of nice to use: the TFT is much better. More intuitive and easier to navigate.
But yes, you can use an old lcd. Personally I very rarely change anyhting via the tft, usually I use octoprint... so on my pc, but just as easily use a phone or tablet. I have an old phone sitting next to the printer that I could easily use with octoprint... simply placing it in front of the printers tft would be the easiest control system substitution ever :)

The printer itself is standard components. You will be able to upgrade ands repair it just like a PC.

Reverse engineering the tft images would be as simple as making a bunch of A, B, C etc images and loading them onto the tft... then compare where they appear with what was supposed to be there... then make new images that represent the actual commands and replace the existing images completely.
But that only replaces the images for existing commands.
I do not know how the TFT's firmware is programmed. AFAIK It essentially sends g-code to the printer, so you would also need to figure out how to add new commands if you want to add new functions. I am fairly sure the tft has separate firmware to do this... not a function of Marlin.