Can I remove catalytic converter, remove loose fragments, then put it back on without harming vehicle? I have a 2003 Ford Windstar.
You can, and it won't "harm" the vehicle, but it won't get much better either.
If the catalyst honeycomb is breaking down, the catalyst element is shot, and needs to be replaced. It will continue to break down and result in more "fragments".
Additionally, if the breakdown occurs inside the honeycomb, those fragments cannot be removed without destroying the catalyst entirely. You will have additional backpressure and poor exhaust flow.
Also, think of it this way: Other than the cost of a new cat (aftermarket can probably be had for less than $200) you are already doing the labor to remove the fragments - might as well just install a new cat.
I've completely removed the catalyst part in the past, but this is illegal on a street vehicle in many places and certainly affects emissions.
Replacing it is the best option, but if the honeycomb inside looks like it is NOT badly damaged. Then watch this video. Here