What causes these magnitudes to be close to each other? Is it because it is a weak dibasic acid?
Sometimes they are close, sometimes they are not. Each case should be analyzed separately.
Why does an isomeric change in geometry from cis to trans affect pKa?
I believe you should be able to google the answer - just look for pages explaining differences between fumaric and maleic acid.
In looking at the Debye Huckel Equation log γ = (-0.509 * zi^2 * sqrt I)/1 + sqrt I, why is it, or not, reasonable to say that the activity of all ions of identical charge will be the same in a solution of a given ionic strength? Note: ai = γi * ci
No - activity doesn't depend on the activity coefficient alone, also on the concentration. Even if the activity coefficients are identical, activities don't have to be identical.
Please note, that the equation you listed is simplified. The more elaborate version takes into account ion radius, so the same charge doesn't guarantee the same activity coefficient.