What stops one of the two following scenarios from happening, consistent with the plum pudding model?
The $\alpha$ particle, attracted by the electrons on the outer shell of the pudding, orbits nearly parabolically around the atom, causing the near-180 degree deflection angle seen.
The $\alpha$ particle hits a plum pudding atom directly, and because the atom consists largely of positive charge, it is deflected by nearly 180 degrees.
Followup: did they know anything about the spacing of these plum-pudding atoms? Did they expect them to be lined up in a grid-like way; difficult to penetrate?
I am a physics undergraduate, I've only taken an intro QM class, it would be nice if that were kept in mind while answering.