To clarify the section numbering thing a bit, Interlude 3 ½ actually was NOT a donation bonus. Wildbow didn’t even set up a donation button until after he’d left college and committed to writing full time (around Arc 6 or 7). Back during Arc 4, he had buffer of unpublished chapters, and he accidentally posted the wrong one at some point, and spoiled some people. Interlude 3 ½ was an apology for that. At the time, he didn’t know that wasn’t going to be the only mid-arc interlude in the story.
I’m not sure this does clarify the numbering, except maybe by indicating that Wildbow didn’t know he’d eventually have to figure out a system that allowed for the potential of multiple mid-Arc Interludes in the same Arc, just in case that issue came up. (That much I’d already figured.) It’s nice to know nonetheless, though.