OK, I've made some changes.
The stat-enhancers now all start at +2, then gain +2 per 6 caster levels. That does mean that they're +8 at 18th level, but they're two levels away from epic anyway at that point and full casters are throwing around Time Stop and Gate, so I don't think it's going to break anyone's game. I also added a secondary minor benefit to each one that reflects the demon it's named after, both to make it a better choice at 1st level (when +2 to one stat is decidedly inferior to other Least invocations) and to give people a reason to take these when they could just buy a stat-boosting item. All of the benefits are defensive, utility, or mobility based, too, to avoid their abuse.
I added the half-fiend option to Servant of the Damned at 11th level.
I overhauled Enslave the Planes to be a one-stop planar survival invocation. While it is still highly situational, I figure people will know whether their campaign will have a lot of planar travel most of the time. Plus, at least it's now a lot stronger in that one situation.