An owl is a symbol of wisdom. That you found the feather first, then the dead owl (if it is a sign) then I'd say the feather symbolizes the spirit and the dead owl the body.
As for knowing that your mom is alright, I'd say she's more than alright. She's in such a marvelous place you couldn't even imagine it's wonders. There is no judgement in that place...only enlightenment I know this because of a NDE, and after having it and having done much research...it's the common thread that shines through even though I knew that before I started my research. My research merely confirmed that because so many other NDErs came to the same conclusion.
Unconditional love is all that matters.