That would add some fun little elements after the plan is executed. No matter what you choose at the vital point :P *Teases* But, yeah, if you want some things to spill over, go for it. However check with me, just so we can figure out if the natures you are taking from Alexi are accurate :P

The chains can retract all the way to the wall, essentially pinning Erik in place, so Joseph's job will basically be to sit on Erik and hold his head still. If Erik is actually at the right point to administer the potion, his blood lust would open his mouth automatically I'd think.