To achieve 1) a single global cutbuffer accessed by both ctrl-c/ctrl-v and
highlight/middle mouse (synced CLIPBOARD and PRIMARY) and 2) the persistence of
the selection after the source window closes, you can use autocutsel and clipit
simultaneously. Running 

    autocutsel -s CLIPBOARD &
    autocutsel -p PRIMARY &

will synchronize CLIPBOARD and PRIMARY, while

    clipit -d &

persists selections. I also prefer to store bit of my selection history, so my
.xinitrc contains a line for clipmenud as well.