You don't have to trust Trump to believe the Russians/Assad were behind that attack. Their own ridiculously lame excuses are about the best confirmation there is.
Some of the conspiracy theories on this don't take into account known facts. The Syrians initially denied they had launched any air strikes at all against that town. When multiple other nations, including the U.S., confirmed that they had, they changed their story to admit that they did. Generally, people telling the truth don't start off with a lie.
The Syrian/Russian excuse was that the facility they hit was actually a production facility that was producing Sarin for use in Iraq. But if they knew that, why not announce they'd hit such a facility, and claim it as a doing a good thing -- preventing a chemical attack? They didn't. And that's because their initial excuses 1) we didn't have any planes there, and 2) there weren't any chemical casualties, were proven false.
The Syrians do still deny that they had any aircraft over that town at the time of the alleged attacks. The Pentagon says it's radar showed otherwise, and actually was tracking the Syrian planes that Syria claims were not there. That is a core factual disagreement.
So, either the "Former CIA Officer" is making stuff up/believing people who are lying to him, or the Pentagon is lying. There isn't a middle ground. And while I don't have a huge amount of faith in some aspects of the government, I don't believe that Mattis on down are lying about Syrian aircraft being in that location.
As for why the Syrians might do it...any number of reasons. One of which may be that they figured they could just blame it on some rebel group and so escape being held accountable.
