Perhaps I wasn't as clear as I meant to be. I meant that the senior most hierarch in the local/national church -- i.e. the senior-most hierarch in the Synod of Bishops for his country/jurisdiction -- which would of course have been Abp. Demetrios. I would expect a bishop to ordinarily have his funeral presided over by another bishop from his national church whenever possible, and not by a priest.
Also, I think that you are very correct regarding the fact that there are probably differences between precedence at a funeral (which does not contain the serving of the Divine Liturgy) and what would happen at a Divine Liturgy.
Again, at a Divine Liturgy, I'm not sure anyone has precedence over the diocesan bishop (permanent or acting) of the church where the Liturgy is being served. In the OCA, when a hierarch dies, I believe that the general protocol is that the primate is the acting diocesan bishop until a new bishop is elected and installed. If this is true in the Greek Archdiocese, then Abp. Demetrios would have been acting both as the senior bishop of the national church and as the acting diocesan bishop of San Francisco.
At any rate, the important thing is that all be done in good order. There was certainly plenty of order in the Vatican this week...
The Church is where the bishop is. All bishops are of equal "rank" -- so that would be, I have no doubt, the way it is: a diocesan bishop would conduct the service.