What evidence is there that he was an "asset"?
Cabell signed a Secrecy Agreement with the CIA. Which means he knowingly shared secret information with the CIA.
If you're a CIA asset, you in some capacity, work for the CIA.
Wikipedia:
In intelligence, assets are persons within organizations or countries being spied upon who provide information for an outside spy.[1][2][3][4] They are sometimes referred to as agents, and in law enforcement parlance, as confidential informants, or "CIs" for short.
There are different categories of assets, including people who:
Willingly work for a foreign government for ideological reasons such as being against their government, but live in a country that doesn't allow political opposition. They may elect to work with a foreign power to change their own country because there are few other ways available.
Work for monetary gain. Intelligence services often pay good wages to people in important positions that are willing to betray secrets.
Have been blackmailed and are forced into their role.
Do not even know they are being used. Assets can be loyal to their country, but may still provide a foreign agent with information through failures in information safety, such as using insecure computers or not following proper OPSEC procedures during day-to-day chatting.
https://en.wikipedia.org/wiki/Asset_(intelligence)
The two documents are a secrecy agreement and a standard 201 personality file (Oswald had one of these too). Neither indicate - and Newman doesn't show it - that he "worked" for the CIA or was an "asset."
This is, frankly, typical John Newman hyperbole and exaggeration. He's simply not, for me, a trustworthy historian.
Newman was a high ranking Intelligence officer. With all due respect, Newman is more qualified to understand the Intelligence Trade than you.
If you need a confession from the CIA that the brother of a high-ranking CIA official was a CIA informant or asset, then fine. You can wait forever if you want. I'm not here to convince you.
What exactly is the allegation here: Is it that Cabell, an "asset" for the CIA was therefore willing to join up with the CIA and commit treason, murder the President and frame an innocent person for the act? You think that because a person is an "asset" (assuming he was) that therefore WHATEVER the CIA told him to do he would do?
I noted the new info on Cabell because it seems unusual and it's connected to the city of Dallas. I could be wrong about it being unusual. Maybe it's perfectly normal for local politicians to conduct secret business with the CIA.
What I haven't done is suggest that it's meaningful as evidence of a Conspiracy.