I'm well aware of the game mechanics and how interrupts work. Doesn't a merc have a cast time for their interrupt? Also, pre 1.4, if Denova was meant to be like this, how would it be possible?
No, and no class has a cast time for their interrupt ability.
EC HM is still tuned for a full Rakata-geared group. After all, that's how the initial people back in 1.2 did it, why can't anybody else? On the other hand, most people tend to get stuck during certain bosses so they continuously do what they can and get the gear from the bosses they can kill until they can pass the checks being thrown out by the game, whether it be DPS, healing, or overall endurance.
However, Rakata, Black Hole, and Campaign gear does not affect what you now have to do during droid phase.