So, my experience in case it helps anyone else. Disassembling the mini was pretty clear from Apple's new docs. I carefully marked all the screws etc and stuck them to some paper to keep clear what was what. This covers everything except removing the heatsink cover, which has 4 screws on the underside with a little adhesive pad on top and little insulating loops. Then 2 pressure clamps also on the underside to remove the heatsink itself.
Removing the underfill round the edge of the NANDs was not difficult, though I did manage to scrape a little solder mask on the edge of the PCB so watch that.
I wasn't getting much traction getting the flat underfill tool under though even at 380 deg C for about 5 mins. I upped it to 400 deg C then was slowly able to remove a NAND, using very low force. The resistors next to it stayed in place without issue.
Unfortunately about 50% of the pads are ripped off. As
@dosdude1 wisely said above, practice first on several other boards with underfill. Did I? No, of course not, I just practiced on lots of plain BGA chips
I suspect my hot air station is under-reporting temperature or something since that time and temp should be enough to melt the solder and soften the underfill. It's a fairly good hobbyist one (Quick 861DW) and I had it on the preheater fairly low so as not to melt anything (just 130 deg C). So perhaps while I was aiming for 'cutting cold butter force' I was applying 'rip off pads nastily force' and most likely it was too cold. 🥲
So, I've got a few spares if anyone else wants a go at also breaking their Mini send me a pm! Two unused KICM223 chips which appear to be in the original packaging, I opened them to look at them with anti-static precautions, but they were sealed. Oh and the rest of the Mac mini maybe, though I'm tempted to keep the pretty case for another project which could still provide more fun... 😀