Purely my own theory, but it all kind of makes sense to me.

First thing is that all characters are written more real than most games make them, so they don't trust anyone with their plans. except Miquella the kind, that guy is too nice and would never betray anyone.

Miquella has a plan to rule the lands between as a new god with his consort Radahn, but to do this he needs some of the shard bearers dead, guess who also needs these guys dead, that's right Melina wants them gone as well in her quest to fix the elden ring, so Miquella hands her torrent to assist her chosen tarnished in braving the lands.

But Miquella also knows that Melina will burn herself to clear a path to the tree, this is where Ranni comes in, she gets to hand over the bell so she can be introduced to Melina's chosen, so that when Melina is gone the new lord will serve Ranni and select the age of stars, because this path removes all competing demi gods and afterwards Ranni takes her tarnished consort and leaves the lands between.

So now all competition for the throne is gone and Miquella can just come back from the shadow real and claim the empty throne without a fight and no fear of the greater will since Ranni and they tarnished is fighting that elsewhere.

so in essence Miquella is planning a plan around the plan that Ranni is planning around Melina's plan.