This is still ridiculous.
Since we just don’t go into a church regardless of the reasons, the rabbis in question should absolutely not publish a responsa.
A Royal Coronation is a rare event and only a few Jews will ever be invited to attend one. Why write a precedent that any clergy could freely dispense to their congregants?
You really have to decide what you want out of the issue before you blah-blah. Even if it’s not your kind of rabbi.