I have to agree, treating homestuck like it was purely made of the fly is an incorrect to view the process of writing a serialized work. When you write a story even if you don't have a written plan you are still keeping track of plots threads and the ideas you will want to explore further in your head. Hussie says as much in his formspring, she had a plan for homestuck that primarily existed in her head. I also want to specify that in the formspring posts Hussie talks at length about the serial vs archival reading and bassicly says homestuck isn't "designed with being read serially, its designed for both". I just think its odd to put such an emphasis on homestuck being produced serially as a big important thing that needs to be taken into account when like, plenty of tv shows that release serially aren't viewed under nearly the same level of scrutiny as homestuck is in regards to the effects serialization has on them. By the start of act 6 it is pretty obvious Hussie had the rest of the story decently planned out, Aranea even alludes to the retcon or such power to ignore doomed timelines to revive Vriska specifically (very vaguely) in act 6 intermission 2 to terezi. Terezis character arc was planned out that long ago. I also want to note Calliope explains the ending to Jane in the very first conversation we see of them.