How do we know which factor is most important to evaluate whether a reaction will proceed through SN1/SN2/E1/E2?
I see this question has not been addressed for some time....so I will stick my neck out...
The point is that YOU DON'T KNOW. And for this we apply
Mix up two organic molecules under different conditions, with different reagents, and anything can happen, and if fact you are likely to get SEVERAL products. You do the experiment, note the RESULTS, and THEN try to rationalize the results. And then see if you can extend this rationale to related experiments; i.e. to see if the rationale has any validity. Chemistry is thus an experimental science.
As a generalization, we might assume that less sterically encumbered electrophiles, i.e. methyl and primary alkyl halides, would tend to undergo associative reactions, whereas the bulkier electrophiles, might be more prone to undergo dissociative reaction. But what do we know? We do the experiment first.