“The proposed object would take the form of a giant, donut-shaped mass of hot, vaporized rock spinning around a molten mass left over from a planetary collision.”
The vapour cools so solid extremely rapidly. My first thought is that rather than a vapour cloud we have a cloud of mixed liquid and solid droplets, like the ejecta from a meteorite collision. What vapour there was would accrete onto the liquid and solid droplets as a rime.
“if the colliding body was small enough, the bits left over from the impact would rain down on the protoplanet like meteorites do on present day Earth. If the object was big enough, it would shatter and form a disc that would orbit for a time around the planet like the rings of Saturn, before spiraling in and increasing the planet’s mass.”
Yes. In Earth’s case the orbiting material coalesced into a Moon before forming a ring, in only a few years. Earth’s Moon formed in a remarkably short time.
“if the two planetary objects are more or less the same size, the collision would be cataclysmic, with both destroyed to form a new molten core surrounded by a huge, thick disc of molten and even vaporized rock and debris – the synestia.”
OK. Not Earth, as it was more of a glancing blow than the direct face on impact necessary to form a “synestia”. It’s an interesting idea, though.
“lasted about a hundred years”
Less than that. Hardly worthwhile giving such a short-term astronomical phenomenon a new name.