Well, there are some strong suspects. Like Mr. Gerry Patrick Hemming. He I believe was a self proclaimed shooter. Another suspect is "Saul" who was given that name by Hugh McDonald(See "Appointment in Dallas").
Saul has a very interesting story. It is claimed by Mr. Hugh McDonald that "Saul" was the fake Oswald in Mexico, captured on film by the Soviet Embassy camera(See W.C. Exhibits). 
Now, it could go both ways... but if I were to say who I would think was the most prime suspect, I'd go with "Saul".
However, according to the "Saul" story, Oswald was fireing diversionary shots from the 6th floor window. Based on ballistics, we know this cannot be true, and at least 1 shot had to of come from the TSBD Snipers nest that actually hit an occupant of the limo. The myserious Saul also claims that he was to kill Lee Oswald right when the shooting was done, but Oswald stepped back into the shadows, and escaped.
As fare as the shell is concerned, I believe there is a photo of it. I will try and see what I can dig up, and I'll get back to you.
The roadsign hit has always been a theory. But according to Jim Marrs, he has seen and original "Garrison copy" of the Zapruder film. A first edition, without the splice at 208. He claims that it clearly shows a missed shot hitting the Stemmons Freeway sign. He then went on to say that the sign was removed early in 1964 by unknown peoples. The highway group claims it wasnt them. The Dallas road service claim it wasn't them. Nobody is talkin about it. But it did dissappear. He claims that it was clear as day that a bullet struck the sign. 
I would be already to believe Mr. Marrs. However, I find it kind of hard to believe that someone would miss so far off. I mean lets speculate... to hit the sign from the TSBD Oswald Window, you'd have to miss by a longshot. To miss by the Dal-Tex building you'd have to miss by a longshot. To hit it from the Grassy Knoll fence, you'd have to miss quite a bit. To be a Records building sniper, you'd have to miss by a longshot.
I'm not so sure where this bullet hole could of been caused... Possibly the West window of the TSBD... that is, if it is true.
Elm st. miss....
If you have an good ol copy of "Rush to Judgement", Mark Lane cites a few witnesses who believed a shot had hit Elm St.  I cannot remember the specifics, but I believe there even was an officer who claimed to of seen it. When he went to investigate weeks later, the miss had been filled in, and not a trace was left except for brand new concrete. I fully believe the witnesses to be credible, and feel that this is a for sure miss.
The floorpan is a mystery to me. A few people on this board could possibly know more than me. I believe I have heard Mark mention it before. He may have info on it. Or even Wim.
The manhole cover.... Well, it could have been caused by a missed shot. But I don't know of many witnesses who saw such a thing, nor any investigator on site that actually believed it to be a miss. I know there is a photograph of the manhole cover being looked at and gthere is indeed a dent or knotch in it, but I believe it would be impossible to confirm it as a miss. However, it cannot be ruled out, simply because it does indeed have an appearance of a possible knick from a bullet or fragment.
http://www.jfklancer.com/photos/DPlaza/plaza3.gif 
Lancer had this labeled as "Manhole cover", but  I still think it is difficult to say. They maybe looking for skull fragments, or bullet fragments.
That's about all I can say about everything... I will try and look up some more, but I doubt I could say anything else.
I haven't completely rules all these missed and such out. I just am still at a mindset of disbelief, and doubt of everything. From both the LNer's side and CTer's. I don't trust much anything anymore. Disinfo is spread by both parties. I onlybelieve what I wish, and that's all any of us can do in a case like this.