Going by these numbers means that if the flower is simmering in the butter for 2-3 hours it will be fully decarbed as well. So probably the additional oven step is not even necessary if you have the time.
With the sous-vide machine it's probably smart to sous-vide the flower right in the butter, that will then decarb and extract in one step.
For a quick result it's probably best to do the 250F for 30 minutes in the oven and then just another half hour in the butter to get all the resin out. What
@cindysid does.
I have an oven with very precise temperature control, so I left the flower in the jar where I decarbed it @250F and then just put the butter into the same jar, set the oven to around boiling temperature and left it there for a couple of hours. That was the least messy.
The jar was closed to avoid smell. That worked fine.
Next time I'll use a bigger jar and add plenty of water, to get the chlorophyll and solids out.
As far as not grinding goes: even the resin "inside" the buds will all go into the liquid eventually, since the buds are soaked in fat and they're simmering for a good while. I guess it would have to be a huge bud and not enough simmering time to "miss" any of the cannabinoids.
So I don't think you have to grind the flower at all, just break it up a little.