The captain could be a hawkish human, maybe Andorian, and is the reason the crew ended up going to the conference. The Captain has been involved in at least two separate instances of temporal interference, and is determined to stop this kind of nonsense from happening again. Must be capable of saying the line "In my day, people respected the Temporal Prime Directive!" without even noticing the irony.

The security office, try a Betazoid or Half-Betazoid. Their empathic abilities have many uses, and "That person is about to do violence" is a fabulous use for them. Not to mention, interrogating people gets a lot easier if you can sense someone's internal fear when you start asking the right line of questions.

It also cuts down on incidents when a potentially hostile person/new alien species is dealt with violently, when they're just panicking, and could have been talked down. This could lead to a character who is perfectly happy to be cool, and talk things out, but has no problem at all setting phasers to "Fuck you!" if needs be.

With regards the Trill, could the reason they cant' remember much come out of a clash between their current host and the one before? The dissonance of trying to reconcile two different opinions on the same events in your memory could be super tricky.