i have never tried using LN2 to remove the die so idk if it's a good idea. you can use a blow torch to melt the solder bumps that connect the die to the pins or you can sand through it. i think the fumes when you do this are toxic so be careful. do it in a fumehood or outside b/c it stinks.also i have not deprocessed a cpu past p4 so i cant help get off the IHS on newer cpu's.

after this you need to do is epoxy the die to a hard flat surface then sand it with 2000 grit sandpaper. before you try doing it to cpu's practice sanding skills on DRAM's. it isnt going to be perfect because humans cant sand down with 1 micron of accuracy. a good way to test how well you are sanding is by how many levels you have exposed at once. if you are 3 or over be careful. look out for cracking. this will be your number one enemy because it will only get worse and bigger die are more prone.

i'm kind of confused with what happened to your thuban. it looks like you ripped the bottom silicon layer of the wafer off and the transistors and metal layers stayed soldered onto the packaging.