I suggest using a long
I suggest using a long skewer, for a road bike rear wheel. This will be 30mm longer than the skewer in the rear of the Silvio. You will need a spacer, ID about 6mm and OD matching the nut of the skewer. ID = internal diameter, OD = external diameter. The length should be 30mm. You could use two 15mm spacers with the mount in between. I would prefer this arrangement personally (because it is better to have the mount close to the frame.
I am not able to know definitively what the stresses will be and if the carboyoke will support them. Towing a trailer with silvio has not been studied in any depth, as far as I know. What I imagine is added tension to the left side of the carboyoke, and depending on the rearward pulling force of the trailer, possible compression on the right side. The carboyoke is great for tension. For compression it is also good, as the front chainstay is under compressive loading to resist the tension in the chain.
The heavy duty rack for the Quest supports at least 20kg. And it is a suspended load, so peak load is minimised due to the suspension. I can't know the true capacity without engineering tests, so we put a conservative figure on it. The heavy duty rack is not specifically a rack for carrying a child seat, again because such a device requires additional testing and validation, although that is how I use it with my three year old daughter.
This is a qualified response, the best that we can do as a small operation.