Unfortunately for the finest-show-that-has-ever-aired-on-television, I believe these to be plot holes.
As stated in season two, an annual Christmas tradition by Newport Beach youths is to destroy the banana stand by throwing it into the ocean. Seeing as how this occurs each year, there is no rational explanation for "250 cc's of [Michael's] father" to be stored there, nor is it a prudent place to store $250,000 lined within its walls.
While it is true that both a drink cooler and (presumably) cellophane-wrapped quarter of a million dollars can withstand being dumped into the ocean, it definitely couldn't have survived a fire nor go unnoticed when run over and be rebuilt; it still seems more likely that it was a writing oversight than a calculated decision by Lucille and George Sr. to keep these things there.
....then again, how did Michael not look inside the decoy cooler before giving it away, too!?