Hello, Mr Smith.
1 - 2
2 - 7
3 - 2
4 - 6
5 - 5
6 - 3
7 - 4
8 - 8
are my marks.

My mistake scale (highly subjective one) that helped me to rank the examples:
1) wrong use of cases, which leads to wrong or no endings;
2) wrong prepositions;
3) wrong word-fomation (были взрывлись ex.5);
4) serious spelling mistakes (исползовала ex.3, "любитецы" )
5) wrong word order
6) not quite appropriate word in the context (Один вопрос, который я считаю очень важным ex.4);
7) non-serious spelling mistakes which can also be misprints (потому чио);
punctuation mistakes
9) cases where most of Russians will make a mistake;
10) cases where I won't be sure whether I'm right.
Of course there's also "mistakes that I don't see", but they can't be included in MY scale of mistakes.

With regret, I must admitthat no Russian would nesitate even a minute in deciding that those texts were written by non-Russian people.