Do you accept them? Do you have conflict of interest or confidentiality policies?
Do you ever have conversations with existing YC companies along the lines of "We had an application recently for product X, you should do that."?
As for the fourth, of course not.
Please forgive me if the question seems redundant. I am reluctant to even ask as YC has a reputation of integrity. However, the mantra that "ideas are worthless" causes concern that perhaps precautions might not be taken to avoid such a scenario (because by implication, it would seem this could be viewed as a non-event). My apologies if I have misunderstood the views of YC on the matter.
As a counter point, if you want to raise external funding, you probably should accept the fact you will likely have to share your idea without protection - as its unlikely any VC will sign an NDA, and you have no idea who they will push your pitch deck to in order to get a 2nd opinion.