Do yourself a favor and just accept the fact that these little buggers WILL break and that you will need to find replacement nylon screws and bolts. A quick google shopping search for "Mac Mini Nylon Screws 4-40 X 3/4" should pull up what you need -- I found an ebay listing for $3.99 USD!

Even if you somehow manage to keep the pins intact during this step, realize that getting the heat sink back on is an even harder task in my opinion.

I found it easier to have another individual assist here -- one to securely push down the pin on one side and the other to firmly grip and squeeze the barbs. Don't underestimate the strength of the springs. The pin and spring can go flying if either end is not secured properly.