The Lusitania was believed to be running guns and supplies to the Entente.
Hardly relevant, given that by their own admission, the Captain and crew of U-20 learned the identity of the ship only
after firing their torpedo, so had no possible way of knowing what it carried.
If the United States is completely out of the war, then there will be less of a motivation for the Germans to sink transatlantic civilian vessels.
Possible, but far from certain. Prewar, GB and France were America's biggest trading partners. No administration is likely to cut off trade with them, and to do so would cause an economic recession liable to cost the President his re-election. Even if actual ammunition isn't sold, the Allies will probably increase their purchases of other goods, thus releasing workers at home for munitions manufacture.
OTOH, a
re-elected President Clark might have imposed tighter restrictions in reprisal for British blacklists or interference with US mails. It is also possible that his term would have been extended to six years by constitutional amendment [1], in which case he has no re-election to worry about, hence enjoys greater freedom of action.
[1] An amendment to that effect passed the Senate in Feb 1913, but got no further due to the intervention of President-elect Wilson. Clark was on record in support of such a change, so would not have done this. Problem is, though, that since the HoR never got to consider the amendment, we do not know what its final form would have been. It might have been made inapplicable to a President already elected, or operative only from 1920. Or the House version might have been rejected by the Senate (which only passed it the first time by a bare 47-23) so that it never became law at all.