There are no serious historians who believe any of the JFK assassination conspiracy theories.
Unfortunately, there are far too many conspiracy ideas out there to take down them all in a single reddit comment, but two just because--
There were a marginal number of witnesses -- 5% -- who thought they heard four or more shots. However, the remaining 95% heard three shots or less; in particular, 81% heard the actual number, three shots. With a confusing situation such as an assassination, there are of course going to be different accounts, and Dealey Plaza where the assassination occurred is echo-y -- this was mentioned explicitly by some witnesses -- so it's not surprising there's a little difference. (Of course, conspiracy theorists focus on the 5%.)
There is a man from some photos known as "Umbrella Man". (He shows up in the movie JFK as supposedly signaling the shooters, despite the fact an elevated sniper would have a much better view than someone on the ground.) We know who Umbrella Man is: Louis Steven Witt. He testified for the House Select Committee you mentioned. He was using the umbrella to heckle the president.
Mr. FAUNTROY. I wonder if you would care to tell us a little more about your understanding of the significance of the umbrella, and why you felt that it would heckle the president to raise the umbrella?
Mr. WITT. ... when the senior Mr. Kennedy was Ambassador to England, and the Prime Minister, some activity they had had in appeasing Hitler. The umbrella that the Prime Minister of England came back with got to be a symbol in some manner with the British people. By association, it got transferred to the Kennedy family, and, as I understood, it was a sore spot with the Kennedy family, like I said, in coffee break conversations someone had mentioned, I think it is one of the towns in Arizona, it is Tucson or Phoenix, that someone had been out at the airport or some place where some members of the Kennedy family came through and they were rather irritated by the fact that they were brandishing the umbrellas ...
Let's take on where the House Select Committee went wrong, and why they made that statement you cite. (More specifically: "scientific acoustical evidence establishes a high probability that two gunmen fired at President John F. Kennedy.") They analyzed police recordings and found sounds which they decided, with high probability, to be shots from the infamous grassy knoll part of Dealey Plaza. The National Academy of Sciences committee did a review -- the "Ramsey Panel". Not only did they find the original analysis scientifically erroneous, they found -- using the cross-talk on Channel II of the recording -- that the sounds identified as gun shots came a minute after the assassination had happened. We know this because the there are voices on the other channel of the recording before the identified sound mentioning the motorcade being directed to Parkland Hospital.
You can read the full Ramsey Panel report online here.
Witness reports are from
Sneed, L. A. (2002). No More Silence: An Oral History of the Assassination of President Kennedy. University of North Texas Press.