All things considered, it seems most fitting for +Sanborn to celebrate the requiem.
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But as we all know, the relationship between SGG and MHT was broken at the time of +Dolan's death. Sedeprivationists were not allowed to receive Holy Communion at SGG. As far as I'm aware, that's the status quo +Dolan left. The priests at SGG responsible for organizing the funeral have a very difficult task in front of them. It does not strike me as an easy decision to invite Sanborn at all-- if he doesn't celebrate the mass that would be perhaps even stranger than if he did.
+Pivarunas might be the celebrant, but Stephen Heiner has been waging war on the CMRI and to most people it appears like a proxy war on behalf of SGG, whose corporate opinion of the CMRI has always been... shaky, I'll say.
+Neville seems the most unassuming and uncontroversial candidate, aside from the Brazilian Bishop (who is imagine is the most preferable candidate).