Most of your questions are addressed by SEVP's OPT Policy Guidance issued April 23, 2010, as follows:
With OPT status, is it allowed to get income from 2 or more US companies?
7.2.1. ... Multiple employers. A student may work for more than one employer, but all employment must be related to the student’s degree program ...
Is AdSense related to my major, computer science?
7.2.4. How does a student show that employment is directly related to his
or her degree program?
SEVP recommends that the student maintain evidence for each job
documenting the position held, proof of the duration of that position, the
job title, contact information for the student’s supervisor or manager, and a
description of the work.
If it is not clear from the job description that the work is related to the
student’s degree, SEVP recommends that the student obtain a signed
letter from his or her hiring official, supervisor, or manager stating how the
student’s degree is related to the work performed.
Unfortunately I can't tell you for sure whether your work to earn money on AdSense is sufficiently related to your major to be allowed based on the above guidelines. Particularly your questions about what articles you can write---there isn't an intrinsically clear dividing line that says what work is sufficiently related to be allowed. The most likely time it might be audited would be if/when you submit another application to USCIS in the future.
Finally, although you didn't mention it, since Google AdSense isn't hiring you as a regular employee, you would be considered as work-for-hire or self-employed and must follow all relevant rules for it. Here's what the same policy guidance says about this for regular OPT (note STEM OPT is different):
Work for hire. This is also commonly referred to as 1099
employment, where an individual performs a service based on a
contractual relationship rather than an employment relationship. If
requested by DHS, the student must be prepared to provide
evidence showing the duration of the contract period and the name
and address of the contracting company.
Self-employed business owner. A student on OPT may start a
business and be self-employed. The student must be able to prove
that he or she has the proper business licenses and is actively
engaged in a business related to the student’s degree program.