Since you don't have paladins (and I would never suggest this in a setting with them), you could give the clerics the angel's healing touch (which cures a bunch of conditions). That would go along way to overcoming the problems with using the warlock as a cleric, since you wouldn't need to have a bunch of "once in a blue moon" spells available (tough with the warlock's limited # of spells that even the limited # of invocations doesn't do much to help with).
Another thought is to borrow from an older warlock tradition (the vestige/binder) and the barbarian to work up a bunch of spirits (and abilities) that the cleric could channel X times per day, so you could have battle channels (possibly similar to totem barbarian rages), utility channels (healing and guidance), and maybe some artillery zapping channels. That way the cleric is a weak warrior until he/she channels. If you did that, you could even do away with spell slots for the cleric all together (channeling is the limitation on how many "spells" they get).