Accounting for the experimental observations.
1. The kinetics.
The first order kinetics indicates that only the haloalkane is involved in the rate determining step. This suggests that the ionization of the haloalkane to form a carbocation in a slow step occurs first. As a consequence, the intermediate (and the transition state leading to it) is less crowded than either the reactant or the product, having three atoms round the carbon undergoing reaction.
2. The basicity of the base.
The elimination reaction requires the site of attack to be the more accessible beta-hydrogens on the "outside" of the molecule. Since the intermediate undergoing step two of this reaction (the carbocation) is of high energy, any base will encounter the beta-hydrogens first and quickly react to produce the more stable alkene product.
3. The leaving group.
The less stable the leaving group, the more energy is required for it to leave and the reaction slows, or stops.
4. The elimination site.
Elimination takes place rapidly with any base, producing mixtures, the most stable alkene in greatest amount.
4a. The stereochemistry.
The carbocation produced in the first step is planar, and has lost the original chirality if present. Conformational requirements found for the E2 mechanism are not found here.