hi viv :)
as far as plating into the horse muck goes, I think that as long as it's well rotted, you can go for it. Until it is, if stuff is planted in it I think it would 'scorch' and harm the roots.
Not sure about rotting of grass turfs, I guess when they look 'crumbly'?
Having said that, my mum and dad made a raised bed for some strawberries from turf edges, to quite a depth, and have had success straight away with that. Perhaps with a bog garden it doesn't matter too much, as the water will help rot it? I wait to stand corrected however! :)