In the Cauldron all crystals are created - both those for Chaos and Planets/Senshi, but also normal star seeds, which every life form requires.

I don't believe Serenity sent them to the Galaxy Cauldron to be reborn though - that process looked and felt different. She seemed to force a direct reincarnation - there was no choice offered to them by the cauldron as to whether they would be reborn or 'retire' with each other, for example. It was more like she put their crystals into stasis until the time was right to be reborn together under the right circumstances.

How would Queen Serenity even manage such a thing?

Is this another Naoko plot hole?

Also, I take it we're to assume Queen Serenity decided to retire?

Do you think Pluto plays a role in the original process of rebirth? Same with Saturn.

Lastly, doesn't Stars seem to go for a finalist understanding of the body whereas Usagi fights against such an understanding?

There's the tricky part where a lot of handwaving or explaining comes in. 

Personally, I choose to believe this explanation:
--Queen Serenity (QS) and Princess Serenity (PS) share a sailor crystal. Theirs is so powerful because it belongs to two people, thus could hold the power of two celestial bodies. QS visited the Galaxy Cauldron before PS was born, so I'm thinking that crystal might have absorbed some of the miraculous powers of the Cauldron, giving it the unique restorative powers that it has.
I believe that QS, who had mastery of the crystal at that point, willed it into being. However, sending the crystal forward into the future to serve as PS's star seed/sailor crystal left her with nothing. No star seed, no sailor crystal. Her soul faded away into nothing, without the solace found within the cauldron.

I don't think Pluto played any role in the process of rebirth, other than to provide the talisman to awaken Saturn, who cleaned the slate.

Stars, to me, didn't really separate their understandings (if I understand you correctly). Galaxia manufactured bodies time and again at her whim and pleasure, using Sailor Crystals to call back the visage of their past owners, showing that crystals and souls were separate from physical bodies, yet those physical bodies had some strong connections to their crystals. When Usagi sacrificed herself to satiate Chaos and end the war, it showed those bodies renewed within the cauldron (at least within their owner's eyes and projections), and (to my understanding) the senshi did not grow again after that point. So the body seems to be connected to the Sailor Crystals during that lifetime, so if they did return to the cauldron and opt to retire, their crystal would move to another host, another girl, whose body would then be attached to the crystal while their visages 'retired' with them.

With the sil mil reincarnation, as they skipped the cauldron step, the body images were still attached to the crystal so they looked like spitting images of their past life selves.