The full context of the passage in Romans 3 is Paul’s teaching of a universal principle – all men are under the Law (vs 19) and unable to keep it. All are capable of actual sins including the ones you have excerpted. Universal principle, but not a universal application.
Were/are killed babies born in sin? David writes in Psalm 51, "Behold, I was brought forth in iniquity."
Very well, therefore you correctly propose to read “all have sinned” in Romans 3 as “all are capable of sinning”. Indeed, Mary, as well as Jesus, the Holy Innocents, Noah, and others were capable of sinning.