I wouldn't dip to anything that slows caster progression. What weapons do you need/ want outside of simple and favored? More specific advice is hard to give without knowing more of your plans and the rest of your party.
With that party composition I would not dip out of cleric, you are the only full caster, so dipping delays the party's acces to full spell levels, and you have lots of beat sticks with you.
As far as diabolist or straight cleric, what kind of alignments are the rest of the group? If the rest of the party is evil, staying cleric for more channel to use healing might be good. Do you think that you will negotiate with devils enough to make use of the diabolist bonuses for that?
For healing he can just pack some wands of CLW
Since Diabolist doesn't drop any CL its great to go into. Heck if you give the imps good umd your healing output can even increase in combat