MINI 5.2 COMPATIBILITY

It is compatible. The original requirements for the Magic Track Pad 2 were macOS 10.11 El Capitan or later, and Bluetooth 4.0. No specific Mac models were required. Both things are covered by your Mac mini, so no reason to believe it would not work.

Yes. High Sierra can connect to the Magic Track Pad 2.

It works for me. I use a Magic Trackpad 2 with a 5.2 Mac mini on high Sierra and a 2009 iMac on high Sierra. Both are working great