Jade -- have you been able to consider the terms by which your firm took
over firm Y ?
It is possible that this includes some provision such as a requirement for
former partners acting as executors to instruct the new firm, or to appoint
"new" partners as attorneys to handle the administration.
Even if there is no such agreement in place, might the executors be willing
to co-operate with you? Obviously, your approach might be different if the
named executors are personally known to, and favoured by, the [main]
beneficiaries - but it is not clear if their views have yet been sought.