the imperial officer elite says that as an activation it lets a figure interupt to perform a move or an attack. Does the unit have to have an activation available or is this in addition to the figure's normal activations?

Since Officer can make a figure move without using that figures movement pool then I would imagine it would work on the same par, the figure isn't activated at the point of you using the Officer so when the Officer orders the target figure to perform the Interrupt then it wont draw on that figures actions for its next activation.

So in effect I could get 3 attacks per round off my e-Web Engineer??

Yes. Cheep thrills for the deployment cost, especially if you can stick him out of LOS. Comboed with a AT-ST is good too. Thematically not sure if he should be giving orders to Vader though unless it's Tarkin!

So in effect I could get 3 attacks per round off my e-Web Engineer??

Yes, that is a good use, I guess Vader really likes to show off when he has an audience around. Otherwise, the officer would be force choked.

Yeah Vader was where it came up. Seemed REALLY powerful as a combo.

Note that officers cannot make Vader use Brutality or force choke. Stun also shuts them off, as the target needs to first get rid of the stun by itself to move or attack.