I believe you got it right there with the path of least resistance - it just went to bypass.
WRT the sand compacting, I believe this happens because many people don't know better or ignore recommendations, however, most pool owners i know are adding all their chemicals through the skimmer since it is teh most "convenient" location.
Now, if you add your bleach that way, it likey will not do anything to your sand, however, floculant could easily clump your sand particles since it's designed to clump fine solids. Other chemicals may also have similar effects, algaecide comes to mind (really, why bother mixing it in a bucket first, isn't blowing it out the return pretty much the same thing?).
If sand can last 5 years+ without compacting and solidifying under noraml use, what makes 7 or 10 or 20 years so different, it has to do something with the chemicals that get put in pools.
I believe adding anything through the skimmer is going to shorten the life span of your equipment to some degree.