Can the Cauldron be moved?

Witches are down right mean with KB blessing, and they do a good job hiding behind crossbows for counter charges. So I would say if you take a cauldron, it is worth taking a medium sized unit of witches (10-15) as a counter charge unit. (or if you are playing against slow block armies, as a main attack unit for R&F)

That being said, one of the best targets for a cauldron blessing is interestingly enough, the war hydra. Since the hydra gets 7 attacks, and the handlers get 3 attacks each, that is 13 KB attacks.

I don't recommend executioners unless you are trying to stay to theme. They just don't have much going for them, and can really only do well when you sink a ton of points into them, or treat them as a cheap throw away flanker. (at 12 points a model, they are bit expensive per model for that...) MSU of 6, assuming you have the FOC spots, is how I would run them. Sure they won't do much, but 6 is a good width for a flanking unit, and you aren't wasting any points on extra ranks that you don't need. Sure if you lose 2 models, they can't deny rank bonus, but in a cauldron list, assuming you kept the unit close, as long as 1 model is still alive, they can hold up a huge block of guys until your next turn.