It's certainly canon, after the discussion toward the end of Skin Game. I'm not sure it was deliberate at the time, for a host of reasons. Here's why I think it wasn't:
1. As originally planned, Molly was supposed to be tagging along, as Proven Guilty and Dead Beat were swapped in publication order, because Dead Beat was going to be his first hardcover, and few things are more awesome and exciting than riding a zombie dinosaur through Chicago. A lot of the conversations between Butters and Harry sound an awful lot like an apprentice asking for more information. Jim slapped Butters in there because he liked him a lot, and having a medical examiner/polka enthusiast along for the ride in a zombie book is pretty awesome.
It still works, of course, and Dead Beat is probably my favorite before Turn Coat/Changes. But it's not really an incident that was in the cards from the start, though that doesn't discount the possibility that Jim rejiggered things when adding Butters in.
2. Jim still hadn't decided what route Harry was going to take at the end of Changes. He wasn't sure if he was going to go with a Coin, pull off a Darkhallow of his own, become the Winter Knight, or whatever other options he could cook up.
3. Butters was being reintroduced to the series as a staple side character in Dead Beat. He was supposed to be a one-off in Death Masks. Because Jim liked Butters and wanted to use him here, I think he was figuring out how he would fit into the series on an ongoing basis.
Arguments in favor of it being deliberate:
1. I think that this particular incident was what made Butters able to become a Knight of the Cross. If he hadn't come charging in right then, I don't think he would've found the courage to do so later on in the series. This was a truly brave choice, and one he didn't have to make. I think it set him on a path of serial bravery that led him to become Discount Batman, and eventually Sir Butters.
2. The previous (only) time we saw Butters was Death Masks, the last time we saw Liver Spots. Butters showed up for the first time right after we first found out Denarians were a thing. It's hard to argue coincidence in a series where destiny is kind of a thing. It's also additionally symmetrical, as it's kind of Butters finishing what he started.
3. Jim is a serial seed-planter. He really seems to enjoy putting in tiny details that don't pay off for a few books.
Can't really think of much else at the moment.
