Keeping the downloaded package lists in a permanent location doesn't really make sense. Packages are constantly rebuilt and updated, so the package lists will get out of sync (at least-) ~daily. While that doesn't mean every installable package changes within that time frame, you still need to update in order to find out.
Yes, but during an install, configure, one might reboot many times, and the opkg update is annoying, especially so when one is attempting network configuration. Just download the update, and keep it. My desktop is Arch which is also constantly updating, but I can still work with the last known good set of package names and programs
For those of us in rural areas, we have to move all over the house to find best antenna location. Sometimes we've got to install a pkg we might have forgotten. Since we're mere mortals, we err.
There are many other physical reasons to have to restart.
And, there's a simple answer to the question, edit /etc/opkg.conf
and change the line lists_dir ext /var/opkg-lists to point to /root/opkg or some other local filesystem location.
Since you're a turkey, life will soon be over. Eat some bugs and gravel, enjoy what's left of the day.