Clovis may have been an asshole ruler, but he still would have helped Lelouch. He kept a portrait of Lelouch, Nunnally, and Marianne in his palace, so he at least remembered them more than the Emperor did.
Cornelia definitely would have helped Lelouch investigate Marianne's murder. Lelouch is needlessly mistrustful of people (except Suzaku, who he has a raging hard on for). Just because he was hurt by the Emperor's coldness, he assumes the rest of his family will be like that. Even Orange, who was the head of the guard for Marianne, felt terrible about it. These people would have done things to help him, but Lelouch is in his, "Well, the Emperor hurt me, I better hurt all of Britannia, BAWWWWWW" mode.