IIRC (and i may be totally off base here) you can set focus then leave the group and your warlock should still have the lowbie as focus target.

As for the tagging:

If one solo toon tags a mob which is then killed by pet or other character and there is a significant difference in levels the low-solo toon will get very little experience. However, if grouped with another toon (like level or not) they will get full experience for the tagged mob. I'd assume that this was to help prevent tag-boosting for solo toons but still allow people to group up to hunt if they're being griefed by a higher level whos just coming along and killing everything they tag.