Banastre Tarleton very nearly captured Thomas Jefferson in his raid on Charlottesville in 1781. What would the British have done to Jefferson? Would he have come back alive? If Jefferson had not been President, would the President in office in 1803 have bought Louisiana from Napoleon or allowed it to fall into British hands? If so, Canada might include everything west of the Mississippi River. But maybe Texas, New Mexico and California would still belong to Mexico.
Probably. It was Jefferson's own principles that made the deal questionable to begin with. I guess it was theoretically possible that some sour old Federalist could have been in the White House and rejected the deal, but that was not likely. It was too good an offer to pass up.