Without taking a gravity reading you can't really know when the beer is done so leaaving it longer is a good way to make sure the yeast finishes up. Within reason, longer IS better.
Yeast will reach terminal gravity on the beer in it's own time depending on how much fermentable sugar is in the wort. Leaving it for a longer period of time WILL NOT cause the beer to "over ferment". That just won't happen.
There are lots of things that can contribute to light body. WIthout seeing the recipe, and the yeast used to ferment it, it's hard to pinpoint an issue. Introduction of "wild yeast" or infection can cause a lower FG and lighter body but would usually affect taste as well.
You are correct with regard to determining FG with multiple readings and not aiming for a specific FG.