Awesome reply!
The beginning of your comment is still talking about the writing system. If you are a kid learning to speak (or in the rarer case that you never learn the writing system of the language) then you don't notice that the orthography is inconsistent. Furthermore, if you are a literate native speaker of a language whose orthography is very inconsistent to begin with, due to an extreme liberal use of loan words, introduced at different times, and pronunciation shifts (I'd say English is a good example), where technically knowing the origin of a word (and when English borrowed it) can help you match its spelling to its pronunciation, you still feel like you're only speaking and writing one language. I'd say an even a stronger example is writing systems that aren't phonetic at all, where no matter what you do you can't figure out the pronunciation from the orthography alone.
Your point about language nerds making up rules about how load words should behave is interesting and true I'd say in most modern languages whose societies contain... academics. That's why you "can't" split infinitives in English, end sentences with prepositions, or say "pendulums" and "octopuses". Unlike German, English doesn't have gendered nouns (pronouns excluded), but still people will try. See "alumnus". The argument about "Zucchini" might exist literally in English, but an analogous one for "Cannoli" certainly exists.
So I'd say your statements about German apply just as equally in English, and so perhaps other people can chime in whether they actually feel like they're speaking a "mixed" language, and at what point they felt like that. In a way, inevitably, almost all modern languages are "mixed".
I think you'll find with all your examples that there is contention. Aren't there German speakers who say "Pommes Frites" the German way and to hell with who ever says that's technically not correct? In the cases that it's not exactly contentious, there's still a funny feeling. Why do people want to say "Computer mouses" so much, even though they utter it and think afterwards "wait, computer mice??". When you take a suppletive form (irregular form) and put it in another context, it s up for debate (academic and probably inside your brain) what you do with it. Not exactly the same as what happens with loan words, but this demonstrates that when you look at language, you'll arrive at multiple answers for how things "should" be, and which one you actually say depends on some decision, by language nerds, or by youself consciously or unconsciously. Language rules are anything but simple!!