So you can use force points to Enhance with successes or advantages. Are these subject to being cancelled, or are they added on after cancelling?

Subject to cancellation.

The idea is you can make the check, and if you're a success or two away from succeeding the check, your can use your force pips to push you over the edge. If you succeed naturally you can dump it into Advantages to get collateral benefits and/or cancel out threats.