Similar things are similar if you're not a specialist enough to see intricate differences. Don't understand your question.

I can comprehend Spanish pretty well and Portuguese can sometimes still trick me for a sentence or two. Languages in particular have very blurry lines between them. Linguists cant fully define what a dialect is either.

Unless you're reading ancient Japanese texts, or looking at an isolated word on a sign or something, it's not really that much of an issue for Chinese and Japanese, even as someone who's never really gotten that far in Japanese. In a longer text, the total absence of hiragana and katakana will generally make it readily apparent it isn't a Japanese text, in most cases, even it there aren't any characters used which you can point to and say "Ah, this is a simplified character from China!"

With Romance languages, either the language is close enough to something I know that I can read it without issue, or it's not and I can't understand it without first studying that language's orthographical conventions and any unique grammar. Still, it never really amounts to more than either "I can't understand this" or "Huh, looks like Portuguese, but I guess it's Galego if they use ñ and spell geografia as xeografía. Well, guess I'll see what it says."

Because Japanese and Chinese characters look similar to those who do not speak/read those languages. I'm sure the same is true for the Roman alphabet.

Because not everyone has developed the schema required to differentiate those scripts or the knowledge base to identify them, and not everyone needs to. This is a non-problem and just about the pettiest complaint you could make.

Don’t get me started on writing Japanese names all in Kanji since that too gets mistaken for “Mandarin” (i.e 近藤浩治) when it’s not.

If you think this example would get mistaken for a Chinese name that says more about your Japanese level than anything else, lol!

I don't understand this issue. Who is misidentifying your writing?