The answer to whether a host for an loan exhibit has sole right to select
someone to officially open the exhibit is "It depends."  Read all of your
contract carefully and slowly, to see if the opening is covered.Sometimes
it is.  Also, your institution has not purchased a product that it can,
within certain generally accepted limits, do with as it pleases.  Your
institution has rented a property that must be dealt with according to
the terms of the agreement.
     Is there any chance of compromise?  A meeting of the two parties
with a goal of "getting to yes" might still be possible.