Seems to me that Eliezer followed the latter pattern when writing Sequences.

I'm not sure that Eliezer outlined the posts in order- he did mention at some point wanting to explain X, but realizing that in order to explain X he needed to explain W, and in order to explain W...

I understand the temptation of posting the outline first, but that's a huge promise you shouldn't make unless you are really confident you can fulfill it.

Agreed. One of the ways I've worked around this is to not post the start of a sequence until it's mostly done (I have the second post to this sequence fully finished, and the third post ~2/3rds finished). I'm not sure I'd recommend it- if you find the shame of leaving something unfinished motivating, it's probably better to post the early stuff early. (I let that particular sequence sit for months without editing it.)