You can not leave Ord Mantell, or any starting planet, without getting your first companion. Every companion is obtained by progressing through your class story. It is impossible to 'miss' a companion absent a major game bug.
They were apparently dismissed somehow, so you have to summon them back.
Not exactly. Both my troopers managed to go to Republic Fleet and gain their advanced class before returning to Ord Mantell to finish their final class quests and then Aric joined them. Happened when they reached lvl 10 and the fleet pass activates at that point. It may be something limited to subscribers, though.