Any way to use disassembled MacBook Pro keyboard and trackpad as input to Mac mini?
In theory it is possible - but there's no on-the-shelf solution that you can go out and buy.

You would either have to make it yourself (requires experience with electronics and programming) or hire someone to build a solution for you. In either case, such a solution would cost many, many times more (considering the cost of your own time) than simply buying a new keyboard and mouse.

The keyboard connector on the mid-2012 MacBook Pro does not feature USB or anything that you can simply plugin to a new computer. Instead it is a special connector, for which you require an adapter board, and the signalling is not USB or anything standard on a Mac Mini - so you would need to create a PCB with a microcontroller or similar to convert the signals from the keyboard into a USB HID device for the Mac Mini.