I can reproduce this. We could fix this by either changing the validation regex in pmb/config/init.py (search for "Extra packages") or by implementing a check around the part where it asks for the extra packages (with a loop, see ask_for_hostname() in the same file).
I think it could be desirable to edit the config file by hand, so it'd be better to do the check at both ends, ie. both when asking for them and when reading the config in.
You are right, having the checks when reading the config file in would be a good feature. Right now we don't do this for the other options, so if we add that it would be consistent to add it for all of them (probably requires some refactoring).
However, adding a check for the trailing comma should be easy.
Yes, taking a shot at one of the above would be greatly appreciated! I'd start with the second, because it's much easier