Hm, while ZUN do make mistakes, just like the next guy, sloppiness still seem a bit too much considering how overprotective he is when it comes to his series? lore and how much of a control maniac he is when it comes to canon material.
When it comes to the core elements of this series, canon is usually pretty solid and well rounded. If anything, official material that are blatantly incorrect or lead to unnecessary confusion are usually addressed directly and either corrected (ie : Reisen?s human status back in LoLK demo (iirc)) or simply dismissed as something that should be considered the same as any derivate work (ie : PC-98, Gold Rush). Now I ain?t no telepath, but I?d say that it might be a bit difficult to take into consideration every single detail from decade old works to the more recent ones. But even so, there is a certain recurring theme, some sort of general outline if you will, that is being respected : there are decades old material that are central to this series that are not only respected, but also explored (most blatant example being the youkai politics, which seems like an overreaction back in SSiB, more elaborated in SoPM, made obvious with DDC ? ISC and actually being explored in FS).
Now then, there are definitively plenty of ideas, points of interests, or simply details that get introduced with each new work. But the thing is? are these part of the core elements of the series or important elements of the work they are featured into ? Or are they just some sort of afterthought who just got thrown in there ? Because if it?s the latter, obviously it wouldn?t receive as much care as this series? core elements : they are pretty much secondary details that ZUN can afford to neglect / not stress over. And if THESE were going to get eventually explored? who says it won?t get clarified ? In any such eventuality, there should be various way of either downplaying potential discrepancies or simply write them off : for the case of the Dragon Palace for example, while it?s obviously based on Urashima Tarou, ZUN could go ahead and have someone say that the guy was completely mistaken, there is a Drapon Palace but he was not there. Likewise for the Shinigamis and the immortals : this issue got downplayed with Kasen pitying Mokou about being someone even the Shinigami won?t bother with, which leads to an open ended question imo. Now, my argument might come like ?it?s minor so who cares about mistakes?, which might a pretty cheap argument, but quite frankly? in the grand scheme of things, minor details?ll stay fairly irrelevant and might not even be explored? in which case overthinking about it would be a rather stressfull endeavor that ZUN can do without.
And when these pesky, stress inducing point of interests DO get explored? it?s straight up, rock solid foundations that you get. The Lunar Capital, for example, was simply described as some sort of super futuristic place and THAT WAS IT, in the past : in fact, it?s illustration in PMiSS (iirc) was more like some sort of super futuristic, sci-fi standards, metropolis than that of Japanese style building. But we got LoLK (stage 4), we even got some precisions regarding how Lunar Capital infrastructure is like in an earlier work (can?t remember the work title, halp). Besides, the secondary details and point of interest are not restrictive, most of the time, anyway : they are pretty open ended and can be molded or tied to any subsequent work with relative ease *cough*CoLA?s content*cough*
To illustrate further on my point on the ?affordable oversights?, here?s one big elephant in the room? artistic discretion. We got no Touhou official manga / artbook / anything picturebook among us so this is a pretty obscure topic, but, going by the comments in the WaHH omakes (iirc ?!) and a ZUNterview regarding SSiB (iirc) here?s what I can gather? ZUN is enough of a control freak to ask what kind of expressions to put on the characters, BUT sloppy enough to have to forget to give Aki Eda precise instruction on how Toyohime?s WMD fan should be designed as ; ZUN didn?t especially care about the mob characters in presence, we actually get Seija chilling with Sekibanki in a WaHH chapter that was released pretty close after ISC got published, in other words, pretty inappropriate for the time imo ; also Momiji?s magic doge parts.
To begin with, the ones who?ll get troubled the most by these kinds of minor oversights are, first and foremost, the lore enthusiast, nerdy overthinkers side of the fanbase : here?s one contentious point (It was a thing, I swear) that came with MoF : Reimu?s amulets working on Gods. Before MoF (and naturally excluding PC-98), there was never any involvement with any got, so Reimu?s amulets, which were generally considered as things that only work on youkai MIGHT be very out of place. How did ZUN tackle this issue ? I don?t recall, but it was more or less ?well, I guess they work?. How do you respond to that ?! That?s a way of dodging the issue? WHILE legitimazing the potential mistake. There are a lot of ideas that fans debate and overthink, to the point where they eventually run into either a question that canon do not necessarily provide an answer to, OR a conclusion that logically requires that there is a discrepancy in canon if it cannot be answered. These are VERY in depth an hardcore, cross referencing , nerdy rationalizations from the lore enthusiast side of the fanbase. Does ZUN takes it into account ? No. In fact, the general consensus about how Lunarians, their amortality and Kegare go hasn?t been reached yet in actual canon but is actually a conclusion from fans (and it?s pretty solid). Point is, some of the overthinking from the fan?s side might not necessarily be in ZUN?s immediaty crosshairs : he do his thing, we do ours, and the thing we do might, hopefully not, be trivialized by it being made irrelevant in official works. Dunno if my point was clear, but, similarly to that, fan reflexions that go too deep into the secondary details and raise questions towards, you guessed it, inconsistencies will not only get trivialized but also become some sort of question that won?t receive concrete answer since they are not being considered.
Personally, I?d say that minor details that official material disregards would fall into some sort of exploitable idea that might or might not be used later. I ain?t no authority, but I say it should be pretty appropriate to simply refer to the ones that are contradictory as ?official?, as in they are official and came in official work but have questionable canon value.
Touhou?s got A LOT of facets, some of which?ll get more coverage than the others, and some of which?ll get might get neglected or simply ignored. You can?t shove EVERYTHING into a small number of work without getting a mess out of it : there are central elements to this series, these have been the general outline for this series, and they are what make the bulk of this series? ?identity?. There ARE inconsistencies, no doubt, but the truly relevant ones get addressed right away by ZUN, while the minor ones, which are mainly discovered by the nerdy part of the fanbase are open ended, exploitable material, and tbqhwy? these kind of stuff don?t really seem to be that big of a deal.
Also, on the ?retcon? thingy? personally, I believe this is, like, the very, very, very, very, VERY last card that an author should pull. ZUN has always been careful and overbearing when it comes to lore, so much in fact you?d see in the fighting games questionable systems that are faithful to the lore. I think carelessly retconning stuff just because they don?t match modern Touhou is? too extreme.
I dunno how impressions go through internet, but I don?t mean to come out as rude or as a some sort of ZUN fanatic with this post. It?s hard to put my finger on it, but there is a certain something that irks me a bit about this topic?