So you seem to be of a contrary opinion, that being that non-pilot/non-CFI (i.e. a janitor) can not give flight instruction,
A non-pilot / non-CFI cannot give flight instruction which can be used for the application towards a certificate or rating (barring some special FAA circumstances).
whereas only a person with a valid CFI can give instruction.
Can only give instruction for the issuance of a certificate or rating as the privileges of their instructor certificate allow.
If that is true, should the FAA investigate the janitor who is teaching someone how to use their G-1000 (assuming he know how to use it).
About the only power the FAA has is to revoke somebody's certificates/ratings. If a janitor is teaching somebody how to use their G-1000 and that upsets the FAA, not much the FAA can do given the janitor has no certificates or ratings.
So what is the opinion of whether or not a person with revoked pilot/CFI certificates can give flight and ground instruction?
Again anyone can do anything they want. It is a very regular/common occurrence for non-CFI's to help out students (I believe it is called a mentor). The time spent with said mentor while valuable does not however count for the application of a certificate or rating.
You need a CFI to get to signed to take the PTS
You need somebody who knows what they are doing to get you to pass the PTS
Two very different things.