You wrote:
1. (the bubble top) could have deflected a bullet, even if not bulletproof,
A very, very slight chance. Read the evidence presented in the WCR. A high-caliber round from Oswald's rifle would have gone right through the plexiglass of the bubble top. However, the bubble top was actually six pieces of plexiglass that needed to be assembled, held together by metal strips, sort of like the way a screen door has metal strips. Had a bullet hit one of these strips, it might have been deflected slightly.
2. The bubble top could have interfered with the sight of the assassin. Possibly, but not probable.
Here's a picture of JFK in the limo with the bubble top installed. Notice that there is nothing but non-bullet-proof plexiglass along the entire rear of the bubble top. You can see the metal strips. Do you think the plexiglass would have interfered with the sight of the assassin? Perhaps if the sun was hitting the glass and causing a glare, though IIRC, the limo was pretty much in the shade of the TSBD when the shots were fired.
It might help to realize that the reason the bubble top was created was so that the president could ride in the limo in inclement weather AND STILL BE CLEARLY SEEN by the crowds lining a parade route.
http://www.flickr.com/photos/that_chrysler_guy/6481321617/in/pool-1848622
BTW - the picture always makes me a little sad, as JFK looks so good and so alive in that shot.
3. There are also reports that a bullet-proof version of the shield had been, or was in development.
Woulda, coulda shoulda. Irrelevant.