#1 ...

sed -e "s/.*([FGH]):\\backup\\/\1:\\backup\\ /" < cat.txt > catall

#2 ...

# (bourne shell syntax, sorry)
for d in F G H; do
  awk '/_/{print "'$d'":\\backup\\" $5 } < ${d}dir.txt
done > dirall

#3 ... could have done this above by piping through sort.

#4 & 5 ...

diff -u dirall.srt catall.src | sed -e '/---/d' -e '/^[^-]/d' -e 's/-//g' -e 's/^/del /' > delfile.bat