While reading that I also got confused between 犬 (which I haven't encountered before) and 太 (which I have). Though that's just my lack of vocabulary. Still, it's pretty hard to tell these apart as a learner since it's not always clear which parts of a glyph are the important features.
My favorite one of these "look the same but aren't" pairs is 土 & 士 which differ only by the relative lengths of the upper and lower stroke. There's also 囗 and 口, which differ slightly in size but more importantly in the hooking of the second stroke; except half the fonts don't display that; and certainly not at the small font size I keep English text at (which is why I've tweaked the font-size for Chinese in my browser). It would be even more confusing when they get used as radicals except 囗 is always used to surround a glyph, whereas 口 is used squished up the "regular" way as in your example.
In the category "differ only by the relative lengths of the upper and lower stroke", there's also 未 and 末.
I'm not sure what you mean about the hooking of the second stroke of 囗 and 口. Do you mean the fact that in the former, the angle tends to be 90° while in the latter it tends to be < 90° (except in fonts, as you note).
I don't know about Chinese, but in Japanese, the former is not a character on its own (at least, it's not in the 2136 Jōyō kanjis, neither is it in the ~6000 characters of the top level of the Kanji Kentei).
> Do you mean the fact that in the former, the angle tends to be 90° while in the latter it tends to be < 90°
No, I mean that the end of the stroke in the former is often hooked and crosses the third stroke, whereas in the latter the second stroke stops short of the third stroke and isn't hooked. I'm not 100% sure; and I'm still learning :)
I don't think it's a common character on its own in Chinese either; I know it as a radical.
Yeah. To be clear; this is what I've guessed from seeing it written in various ways; I don't have an authoritative source.
(In general there doesn't seem to be a good source for what kinds of variations in a character are accepted and what the "kernel" of the character is.)
There is, unfortunately, a great lack on this very topic. In the digital age, it's sad that for those things, you pretty much have to rely on books.
There are a few resources, like glyphwiki or Kanji-VG, that have stroke information, but TTBOMK, they don't contain any information about the kinds of strokes.
Even if they did, that would barely scratch the surface of what I am, personally, interested in. For instance, I'm taking Japanese calligraphy classes, and am interested in the various ways characters can be written in the different styles (楷書, 行書, 草書).
I've also found that interesting etymological facts about kanjis help me remember how they're written. To give a couple examples of things I've found by looking at japanese kanji dictionaries, 並 (line up) is derived from 立 (stand up) repeated twice (立立), and 自 (self) had the meaning of nose before the character for nose was created (鼻), which is why you'll find it in many nose-related characters like 臭 (smell) or 息 (breath). The latter is the kind of thing that I've found most learning apps completely miss. All those I tried, which are all essentially based on the same data sets anyways, will tell you that the radical 自 is for self, and be done with it. Somehow they do manage to tell you the radical 月 can be related to flesh (which is why it appears in many organ-related characters). In the worst case, Kanji-VG based apps will even tell you that characters with the radical 自 contain the radical 目, because you know, 自 looks like 目 with a stroke on top. Which is completely misleading.
Essentially, what I've found works for me is to go to the local library, open good old fashion japanese-japanese dictionaries, and research the subject. It helps that I live in Japan.
I did try a few japanese-targetted apps, but they are mostly drill based, and/or heavily targetted at kids. That doesn't work for me. Maybe I simply haven't found the good ones... finding something on those app stores is so impossible.
As for japanese learning tools for foreigners, they tend to be targetted at beginners, which is fine, when you're a beginner.
On the subject of drills, I haven't found any that actually tries to challenge on similar-looking, possibly even homophone characters. At best, they will make you pick between homonyms, like "given $context, do you write きんし 禁止 or 近視?" (both are correct writings for きんし, but have totally different meanings, so the context tells you which to use), but I haven't found any that makes you pick between homonyms that are purposefully wrong but very close looking. Like "do you write そくおん 促音 or 捉音?" (only one of those actually exists, the other would actually have the same reading if it did exist, but it doesn't).
My favorite one of these "look the same but aren't" pairs is 土 & 士 which differ only by the relative lengths of the upper and lower stroke. There's also 囗 and 口, which differ slightly in size but more importantly in the hooking of the second stroke; except half the fonts don't display that; and certainly not at the small font size I keep English text at (which is why I've tweaked the font-size for Chinese in my browser). It would be even more confusing when they get used as radicals except 囗 is always used to surround a glyph, whereas 口 is used squished up the "regular" way as in your example.