...get the corresponding UML element for a given FigXXX object?

Each FigXXX implements the method getOwner() which returns the appropriate owner element which is responsible for this Fig element.

...get the corresponding Fig element for a given ModelElement?

for this one needs to iterate through all Fig objects and invoke getOwner. Compare the result with the given ModelElement. Be aware that there might be more than one Fig object for a given ModelElement if it appears in multiple diagrams or there may be none if it has no diagrams.