Let's start with the most obvious question; how did no one know that the original SheZow died?
Everyone did know that Guy's aunt had died, they just didn't know she was SheZow. SheZow just disappeared for a while as far as everyone knew. For whatever reason, nobody notices that she looks and sounds a bit different.
The question here isn't really clear. If the question is how did nobody know Guy's aunt had died and She Zow with her no timeline of any sort is given during the show. She Zow may not have even gone missing if the cops could keep things under wraps for a couple days, weeks, months at the most before Guy found the ring. If the question is why don't any of the villains and assorted other people know that this isn't the She Zow they've been facing before I chalk that up to a dumber than average cast.
If SheZow can't remove her ring, how did it get hidden in the wall? Did someone stash it after taking it off her corpse, or can it place itself where the next person it chooses can find it?
It could work similar to the Green Lantern ring and return to the wall upon the owner's death.
It could also be you can't remove it when you're SheZow, as we later see Guy giving it to Kelly in "SheZow for a day".