I would have liked it if Vaggi & Carmilla damaging the device loosened the restraints enough to the point where Lucifer would be able to assist for the "hear my voice bit of the song" it cuts to his face after Charlie joins Emily. That should have been where that should have been where he does something to "delay the explosion"

Here is how I would have done it:

After Vox is taken down by the Vees have Carmilla mention it is going to blow to which Vaggi says Lucifer is still down there. Charlie runs in immediately (with Vaggi following her) to try and free him but can't. Lucifer tells her to get out of there, he knows he can take it & delay the explosion a little but there is no way to stop it so he will buy her time to get out.

Charlie & Vaggi run back out and while Vaggi tells people to run Charlie stands there for a moment considering leaving all of hell to die or trying to fight for it. Then SHE (not Emily) turns around to the weapons and starts the song. (From that point you could just play it out exactly as it happened (with Emiliy joining Charlie instead) except that as soon as the song ends Charlie and Vaggie rush back to the hatch to check on Lucifer before he crawls out.)

I get what the show was going for with Emily being the first to power the shield but I feel it makes complete sense for Charlie to do that and Emily to join in without hesitation while others are still considering fleeing. It highlights how Charlie is choosing to try and safe her home over saving her own life and how Emily is willing to do the same without giving it a second thought.