Tank =/= go in and die. In fact if you are tanking, the whole point is that you are in a robust ship with looooots of engie skills, preferably an engi captain, so you can stay alive forever against a tac cube piloted by Q himself.
Tactical =/= having anything to do with tactics. In STO, it means DAMAGE. As in, doing it as fast as possible. Tacticals go in escorts. Escorts, being nimble, with lots of tactical BOFF slots, and able to mount DHCs, are the PERFECT ship for a tactical to do its job in. (Though, tacticals in other ships works too, play what you want, sucka!)
The game's mechanics do NOT conform to the lore you want it to. Sci vessels and officers HAVE a use in STFs, it ISN'T calling the shots, but it might be. It's mostly CC and support. (CC= Crowd Control, not Command and Control or Cat Carrierin') 1 well piloted sci in a sci vessel in an STF can make it a lot easier to be wreckless (i.e. do MORE dps FASTER in a 'scort) and shave a few minutes off the run time, if you care about such things. Engis in Cruisers are tanks, hands down. THREAT CONTROL = threat. This means the person in the STRONGEST ship, able to stay alive the LONGEST needs to take it. So Engis in cruisers take threat control. Well, anyone in a cruiser built for tanking takes threat control, tacticals will be the weakest at tanking in a cruiser though.
Finally, a sci captain in a sci ship (especially the nebula) COULD be a fairly decent shield tank. I think.