Re ran optipng against a copy of folders.
Time was ~ 2h for each run.
no errors were encountered with/without -np option

Results:
size   | optipng parameters
-------|-------------------
131864 | original - not optimized
131412 | -o7 -clobber -fix -force
130512 | -o7 -clobber -fix
130516 | -o7 -clobber
130512 | -o7 -np -clobber