Learn English – Could you send the datasheet within this week

I agree with Tetsujin; native English speakers differ. I've heard native speakers use both meanings in each of the examples you give. I most often hear "this Monday" and "next Monday" to both mean "the Monday of the following week". And of course contextual cues can be used to determine if the speaker is intending "this Monday" to be in the past. "This Monday the weather was so nice I washed my car" is pretty clearly in the past.

I personally am in the habit of saying "this past Monday" and "this coming Monday" to clearly indicate which I mean.

Actually, No. On the contrary, if the deadline is on March 30th, and a student is late, then he must produce a doctor's certificate and submit it by April 1st.

The deadline is on March 30th. The statement clearly states that in case any student is late to submit the submission, he will be excused only if he submits a doctor's certificate within 2 days.

So he has to submit the certificate from a doctor stating the reason behind his delay by April 1st. ("By April 1st" implies that he must submit it on March 31st or April 1st, beyond which his certificate will be deemed invalid/ useless).

If the deadline was on March 30th, and he comes up late and submits it on April 2nd, it will not be accepted, irrespective of his possession of a doctor's certificate.