It is true that Jefferson was absent during the drafting of the Constitution but since he was one of the most knowledgeable and intelligent men of his time his opinions should be given more weight than that of an average citizen. Putting into practice the theoretical draft that was our Constitution is a significant portion of how our government works. Jefferson had a hand in that implementation. I am assuming that your are a conservative and the unwritten traditions and customs of how our Constitution works are of importance to conservatives and Jefferson had an influence on those traditions.
Yes he did. Jefferson corresponded with Madison, who in turn offered many of his suggestions in the convention. Madison in turn kept Jefferson up to date on the happenings of the convention. His autobiography indicates that he knew extensively of the document's formation and theories during the ratification debate. The fact that he did not campaign in opposition to the document, though he, like Mason, desired a bill of rights, is largely credited in facilitating ratification in Virginia. Madison could not have overcome Mason and Patrick Henry in a ratification fight had Jefferson endorsed their position.