How can I complete the prerequisites for MS in Computer Science? 2019 application
Specifically, I need to have completed courses in Data Structures,
  Algorithms, Operating Systems, Theory of Computation, Compilers and
  Computer Networks.


These are the courses you'll need to complete then, and you'll need to complete them at an accredited school of some sort.  

Professional certifications like CompTIA a+ / Network+ etc. won't cover those topics at all, and simply wouldn't address these requirements. Some of those computer science topics are available on edX and Coursera, but most of them aren't accredited, or proctored, so the school you are applying to may not accept them. Edx does offer an online MS in Computer Science in collaboration the Georgia Institute of Technology, but you'd have apply for admission to that program, and they too will want evidence of having taken undergraduate courses in algorithms, operating systems etc. Also, when taken for official credit, the Edx courses are not cheap! 

That leaves you with option (1). I assume by PG you mean post-graduate. You should communicate with the school offering the MS, and see if they can offer suggestions for how this could be done. 

There is a remote chance that you could learn these topics by self-study and demonstrate your mastery of them to the school offering the M.S., but this is something you'd have to negotiate with the department you are applying to. The department would probably be highly resistant to this because many who claim to be self-taught really don't understand the depth of preparation required, and it ends up being a lot of work for the department for very disappointing results. Many people can teach themselves JavaScript, many fewer people can teach themselves the theory of algorithm design.