This element must be a sibling of the element with the synchronization attribute. I agree the constructs are not equivalent completely. My purpose was to show that the concept is there. We used it in another direction. An important factor concerns causality.
Causality implies that an object can be synchronized to only one event.
A consequence is that an existing constraint might need to be re-edit in case another constraint gets added. Here we start building the SMIL-tree. I don't understand you. If two synchronization specifications are in conflict some resolution scheme is required. If the synchronization is not allowed to be in a range and the object is not allowed to stretch or to shrink, the remaining solution I see is to specify a precedence rule.