Grounding TV Antenna and Metal Porch, And Resolving Ground Rod Issues
Two many questions so I will tackle a few code issues. The water pipe is required to be bonded to your electrical system so go ahead and use it as a second electrode. In truth that water pipe is normally a much better electrode than a standard 8’ ground rod.

I might drop a 2nd 8’ at 6’ or more away from your existing unknown rod.   Many people here probably have not heard of a 5’ communications ground rod but it is normally allowed when the service panel and grounding electrode system is further than 20’  NEC 800.100.a.3  &amp;  800.100.B.3.2 

Section 800 is a stand alone section and is not subject to all the rules of chapters 1-7 as specified in 90.3