Couple things to break down here. The reasonable and probable clause relates to it all. Felony or midemeanor. Direct witness or with "immediate knowledge" relates to both. Direct witness is far easier to defend against I'll grant that. But immediate knowledge is a defense here as well. Greg stated he saw arbery hauling ass past his porch in his statement to the police. He said he immediately recognized him from the footage as well of previous trespassing attempts. I don't know how the argument will turn out in court for defense on immediate knowledge or not here. It can go very badly for the two if the jury don't think it is a valid defense in this scenario. I still find it unlikely given the circumstances, but could turn out that way.
However, you are committing a logic fallacy here. We don't know if the McMichaels presented firearms here. Presenting implies escalation of force. Carrying isn't considered use of force. It could very well be that there is evidence, like witness testimony still not released, that state if the McMichaels aimed their guns during part the chase or as Travis got out. The video we the public have seen doesn't show that though. Doesn't mean it didn't happen since the video wasn't showing the actions of the McMichaels during the entire encounter. I've already stated for misdemeanor citizen's arrest I feel a jury would certainly not consider presenting a firearm as a reasonable escalation of use of force so far if one can be proven to have happened. Lastly though on this point. Even if the McMichaels did aim their firearm(s) at Arbery during some point of the chase, if it confirmed with other evidence we don't know about yet that Arbery had stolen something then dumped along the way, or the McMichaels could reasonably believe (by a jury of their peers) that Arbery was running from a felony and not a misdemeanor (like say Perez told them "He's running because he stole something!") then the escalation of force would be justified by them.
For reference on Georgia's brandishing law:
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https://law.justia.com/codes/georgia/2010/title-16/chapter-11/article-4/part-1/16-11-102/)
Different states have different laws regarding brandishing. In Georgia you have to POINT your weapon and have no legal justification in doing so. The mere act of carrying a long gun in hand in Georgia is not brandishing or presenting there.
As I said before, there is a lot of evidence I know hasn't been released to the public and we'll see what plays out in court.