Does that mean that this individual Praetor (competing with other Praetors) can unilaterally start a war for all of the Romulan Empire to get sucked into?
be a classic thing for Praetors to do. The Roman Empire was constantly in a state of war against all sorts of opponents, especially in the "peaceful" times (where peace arose from bountiful war booty), and it didn't really take the deliberations of the government to get those wars started...
Or is the Romulan Praetor the actual emperor?
This is possible as well, if he's the equivalent of the Roman "Praetor Maximus", the alternate name for the dictator-for-a-day that was elected to solve major problems and ruled sovereign through a crisis. OTOH, Q once referred to the Romulan Empress, suggesting a more straightforward title was in use at least at some point of the Romulan history.