It looked to me as if the two collars with the keyways in them are positioned to coincide with the keys. The blades have to have big enough slots in them to allow the whole assembly to slide over the crankshaft, but they do not need to be fitted keyways - they can be filed roughly, since they are not driven by the keys.
The Woodruff keys should be a decent fit in the crankshaft and the collars. Those are all properly machined keyways, so good fits should be achievable. As you said, if there was a great deal of slack in the keyways, there could be a form of chattering that would hammer the parts, deforming them and causing premature failure. Keys and keyways must always be well-fitted, to work properly.
I do not know what you mean by slack in the welds.
