First, there is something else that needs to be mentioned to the socialites - Bluff, Diplomacy and other social skills do not constitute mind control. That is, there is a huge difference between what a Paladin with a "helpful" attitude might do and what a red dragon with a "helpful" attitude might do. Further, Diplomacy is the art of compromise, and even a very favorably inclined individual might expect something in return for their aid. This is normally considered reasonable, but it should be mentioned at the start of a campaign.
RBG - Yes. Be aware that procurement generally constitutes a significant difficulty. As such, the 1/day limit itself should rarely come into play.
Regarding your other comments - Knowledge (religion) ranks are hardly wasted in the way that Profession (gambler) or other obscure skill points are, and are quite useful in and of themselves.
With regard to the Dark Craft rules, it is clear from the BoVD table that either XP or GP - but not both - can be gleaned from a single sacrifice (just as, say, a cat's grace spell or a bull's strength spell might be obtained, but not both.) Also, sacrifice is not a reliable thing, and the rules text state clearly that even if a specific reward is sought, it is by no means certain that it will be obtained.
Finally, it should be mentioned that sacrifice cannot be done while crafting (as with anything other than eating, drinking, talking, and resting during that time) and that at most one ritual's worth of virtual dark craft XP and one ritual's worth of GP can be applied to one item - and only once. This last restriction is presumably not an issue for the typical cleric, but it may be an issue for an artificer.
These issues are clarified here so that you yourself can decide whether or not this constitutes a viable activity.
SG - Great cleave is rarely used in most campaigns, but who knows?