So glad you embarked on this site, Alan.
Despite a fair amount of knowledge, I am not near as learned as you other early forum members; but I appreciate the opportunity to share. Since this is not a scholarly piece, please forgive me for not citing all the specific evidence in the portions regarding 'who' and 'why' when I use the phrase "evidence suggests".
I believe the truth of the JFK assassination can be viewed on the larger "crux" question (one shooter or multiple) with near certainty: i.e. beyond a reasonable doubt. More than one person was shooting at the President in Dealey Plaza that day. This is proven by the spacing of the (audible) shots. Bang...........Bang-Bang! Impossible to fire that quickly with the rifle that Oswald reputedly used. This is not even arguable to me: the audio evidence confirmed by every Secret Service man on the ground that gave testimony to the Warren Commission, and the majority of the witnesses, no matter where they were in the Plaza confirm this. Those who believe in one shooter, i.e. Oswald Lone Nutter's have never even begun to try to explain this discrepancy seriously. Usually there are generally comments of "echoes" and "unreliable witness testimony", before the subject is delicately changed to areas where the LN'er feels more confident. The Z-Film, which I believe also supports the audio evidence; adds the extra dimension of DIRECTION of the shots. Despite Lone Nutter's valiant attempts to persuade otherwise; a rational person (or a rational person being perfectly honest with them self) viewing that film must conclude different direction's are likely involved.
After this, my confidence level starts to drop on the who and why. For example, although a fair case can be made that Oswald could not have been shooting, and then had time to make it down the stairs to that lunchroom and Coke machine; I am still on the fence about whether he was one of the shooters or was a 100 per cent patsy as explained in one of the above posts. My studies and reading have me leaning toward a patsy scenario; with the caveat that Oswald was not a total innocent; meaning, he knew something was up that day, and he was part of a plan of some kind. Assuming this is true, I believe it became apparent to Oswald very quickly of some sort of "double-cross", as he exited the Plaza during the confusion just after the shooting.
As for the Tippet shooting? There again, I have heard good arguments on both sides; but I lean toward Oswald himself shooting Tippet. However, I do not believe that a random cop (who is not cruising in his usual "beat") happened to see Oswald looking suspicious, tried to stop him, and Oswald shoots him in a panic. Tippet's actions leading up to his death lead me to the theory that he was not an innocent himself; and that he was involved, at least compartmentally in the post assassination scenario, and was following orders to find Oswald for more than just an arrest. Perhaps they even knew each other. It is possible that, in Oswald's mind, shooting Tippet was literally (as in kill or be killed) an act of self-defense.
Oswald at the theater? I lean toward the theory: prearranged meeting that never happened; but it's possible he just picked it as a last minute attempt to lay low until he could figure out his next step. An early weekday matinee was not in his favor. Not many people to blend in with. Then again, eye witness reports of Oswald's actions after he arrived at the theater may suggest he was looking for someone.
Since there is no reasonable doubt it was a conspiracy of some kind (more than one shooter), who is behind it? The evidence suggests factions of our own government via the C.I.A. decided at some point in 1963 that killing their own Commander in Chief, was vital, for a variety of reasons: the perception that JFK wanted to diminish the "Industrial Complex" i.e: make the USA weaker militarily; combined with his failure, in the last minute, to support the Cuban invasion against Castro...you all know the arguments. I do not believe it was mainly a Mafia hit, as some theorize. But, underworld figures cooperated fully in aiding and abetting the plan. Their motives were obvious; Bobby Kennedy's crusade against them with his brother's support; not to mention their business banishment from Castro's Cuba, with no help from JFK. And of course, the (factions of the) C.I.A. found it easy to recruit Cuban dissidents who hated JFK for what they believe was his abandonment at the Bay of Pigs. Many held him personally responsible for what turned out to be a slaughter.
As for the F.B.I.? Evidence suggests Oswald was a low level informant for them. The C.I.A. took advantage of this fact to manipulate Oswald in some way. Hoover and the Feds were only involved after the fact in a cover-up to save Bureau face; or more charitably, because, like Johnson felt, the Lone Nut Myth had to be perpetrated to avoid a possible war.
Other than the question of conspiracy, which I feel is settled, I am open to be persuaded on other areas, where I am not so certain. Let the journey continue...