Re: #3) Yes, you risk shaving metal into the engine, but yes, the filter should remove it. Still, you never know...

Re: "If the threads get stripped, is it usually the threads on the plug or the threads in the pan or both?" Good engineering practice would be to make the smaller removable piece (the plug) out of a softer metal than the larger and harder-to-remove piece (the pan) so that the plug gets sacrificed. But I don't think they always do that.

Examine the threads on the plug closely - use a magnifying glass if you can - and see what there is to see. Are there some threads that seem crushed in? Are there some that look like uneven compared to the rest? Do any show a glint of fresh metal? If you've stripped the threads or cross-threaded it, you should see some sign of unnatural wear.

If the problem is the threads, a plastic washer isn't going to help. You'll need a new plug.

Good luck!!