Some of it still isn't clear. In fact, A LOT of it still isn't clear. I doubt Lucille 2 was alive after Cinco. The telephone calls that Michael kept receiving from her were pre-recorded messages from the Lucille Austero campaign, something Michael wasn't smart enough to figure out as he never listened to the calls the entire way through.
One theory is that Lucille 2 was "the happiest corpse" because she and Buster f*cked and then she had a heart attack. (The "happiest corpse" is a lyric from the song Cabaret, from the musical of the same name, btw, and the last episode ends with an instrumental version of the song; Liza Minnelli starred in the film version of Cabaret). So her death could've been entirely non-murder-y, but Buster would've felt responsible or that he was supposed to take the blame (like he did for Mimi's death, even though he actually did push her down the stairs).
I'm still confused by what happened at the stair car. Was Buster genuinely surprised that her body was disappeared? What happened on the five minutes of videotape that were deleted? Did Fake Block actually do that?
On the other hand, we've seen through out the series some of the things Buster's done. And maybe he actually was a monster like he kept saying!
Another possibility was that her brother Argyle killed her - but there's no, err... concrete evidence to support that.
Buster then hid her body but had difficulty digging her up for the wall trick - which he alludes to. He used her body in place of the mannequin - maybe the same one Tony Wonder substituted during the closet switch trick. This would explain why Buster used Lucille 2's body in the wall instead of a mannequin: he'd dumped it in the ocean (which ended up getting caught on tape).
Ironically, Lucille 2 didn't want to build the wall but she did!