I agree with you fully on the first point. For the second, I also think OP could've phrased it better but in my opinion (as a simultaneous bilingual who has also acquired multiple other languages) what they meant was that you don't typically forget super common words in any language. In fact, your typical domain of use for whichever language influences how easily you can retrieve the words in each of your languages. If you discuss work often in English but never talk about it in German, you'll struggle to remember work-related terms in German. If you only talk about your work in English then you might forget some everyday words in it and struggle to hold a 'normal' conversation after a while. Basically these things are domain dependent. But it's basically never a good example to have a character forget the word for "hello" just to illustrate they are bilingual.

Of course if you're surrounded by people who speak the same languages as you, you might borrow words from each of them to form your own idiosyncratic code. But the thing is, 'spanglish' didn't arise because people forgot the words in spanish or english but because it is an expressive way to communicate with people in their community that a) emphasises belonging to a specific group (like all languages/dialects) and b) makes full use of the linguistic faculties available to the speakers. Like, why would you restrict yourself to one language if you don't have to? In my experience this kind of borrowing is really more about adding to the language(s) than forgetting words.