Regarding your first question, acceptance into YC definitely carries a lot of weight and can go a long way toward supporting an O-1 petition. Regarding your second question, as long as you are not receiving any value for your work (with value not being limited to cash compensation), then I see no issue with your release of a free app to help others.
Follow up question based on "value not being limited to cash compensation"
So it'll work well until getting accepted into a program like ycombinator, but I'll have to explore the O-1 visa options after getting accepted? Thanks.