I'm doubtful about Garner playing Miss Marple, Jennifer just doesn't seem to have that special something which the previous ladies do have or did have.
I'd rather Dame Judy Dench for a feature length instead of a young one

Can't see a young Miss Marple being successful unless the director scours the UK for that very special talent that brings the youthful Miss Marple out.
Good luck but I'm not holding my breath.