Learn English – Turn To, Turn Into

I think you're overanalysing this one. Obviously native speakers often make "slips" like this, especially in casual conversation or Q&A website comment posts.

In the case of X mistaken for Y, I imagine almost every competent native speaker would say for is "correct", but many if not most wouldn't see anything seriously "wrong" with as. A (probably, much smaller) number might not see much wrong with with.

This is an example of a comparatively minor slip. On a site like ELL, for example, many users are native speakers, and many aren't. As it happens, we have contextual information for the specific comment that sparked this question, making it highly likely that the writer is in fact a native speaker. But even if there were no such evidence, the fact of the usage itself would be no real reason to suppose otherwise.

This shouldn't be taken as a general principle for all "misused" prepositions, etc. Some errors are far less likely from native speakers, and far more likely to be seen as "giveaways" indicating "non-native".

Native English speakers generally use 'Cover' in a sentence while referring to paying a bill or supplementing somebody else's part of the bill.

To be honest, as I'm so used to saying it, I'm not 100% sure if it is 'correct' to use but it is definitely not incorrect.

An example of me using 'Cover' in a sentence:

Can you cover my part of the bill? I will pay you back as soon as I get the money.

Don't be afraid to use it in a sentence as it makes perfect sense and everyone will understand what you're trying to say.