I actually think those kinds of mistakes are made more often by native speakers, because they learn it from other people as they're growing up (including all the mistakes), while non-native speakers learn it correctly (from books and teachers). Same goes for the then/than or they're/their/there, etc. When you learn it spoken first, and incorrectly, it's harder to correct those mistakes than to learn it correctly from the start.

In Dutch, for example, we have loads of people who will say "groter als" (bigger than), which is dead wrong - it should be "groter dan." This als/dan-mistake is something typical of natives, and I've never heard a non-native make this mistake. Same goes for zij/hun. Usually kids just learn incorrectly from their parents. My own parents make those mistakes as well and it took more than a year of my elementary school teacher correcting me every. single. time I made the mistakes, for me to correct them.

There are languages that are considered isolates, meaning they are spoken and written languages that share no connection to any other known language. Basque is an example for that, spoken in the Northern parts of Spain, but does not share any similarities to Spanish, Portuguese, English or French. Common theory is that it just developed in that region and was not influenced by outside factors at all which is linguistically weird since trading with other regions was common. There are also no known languages that descended from Basque, thus it's not a proto-language either.

Less vs fewer is definitely a mistake made more by native speakers, who may have known the words most of their lives without a defined meaning until later.

This grammar mistake is one of my pet peeves with online chat, and really seems to be getting worse in the last few years.