Imagine Buell on the stand saying 'I wasn't really paying attention to the bag'
And let's see an Oswald motive for shooting Tippit if he hadn't also shot Kennedy.
So you're guessing Frazier would have caved under the pressure of the prosecution--with his friend Oswald sitting there in the courtroom? I'm guessing the opposite. That Frazier--who has been consistent in his belief the bag he saw was roughly 1/2 the size of the bag placed into evidence--would have been even more solid on this point.
As far as the Tippit killing...we really don't know what would have happened. For all we know his defense team would have gone right after Tippit--and perhaps even tied him into the plot. In such case, Oswald's defense team may very well have been able to mount a convincing case for self-defense. It's far too gray.
As stated, it seems probable Oswald would have been convicted should the prosecution not pile on. The more they piled on--by, for example, presenting evidence about the noble character of Officer Tippit, who may have had all sorts of skeletons in his closet--the more they risked losing the jury, IMO.
I mean, can you imagine the prosecution putting Tippit's widow on the stand to testify to her husband's integrity, only to have the defense whip out that he'd been unfaithful to her? (Which may or may not be true--which is kinda the point). The bottom line is that with probing enough questions about Tippit's character and behavior may have been raised to sway a jury away from what might otherwise seem obvious--that Oswald killed Tippit in the first degree.