Well, we don't know if the missiles were severely damaged or dropped in the sea last time, the Russians' words "liquidate" can be interpreted in many ways, who knows.
With such expensive and highly classified missiles, the ship carrying them would 100% be special chartered, and was probably carrying dedicated protection details. This is supported by the fact that the transport ship headed back as soon as the accident happened.
There is simply no way anyone in their right minds would stick state-of-the-art missiles on a commercial container ship, and such valuable missiles would not be stored on deck.
As such, the chances of any of them ending up overboard would be zero in my view.
I think it’s safe to conclude that the last shipment was damaged, either through flooding; falling of other similar mishap; and the Russians scrapped the lot.
As for why China asked for Sea freight, well I think it’s probably to do with opsec.
With rail transport, those missiles, TELs, radars and associated equipment would be clear for all to see even if you cover them with tarp.
The high visibility and low speed of rail over such long distances means it pretty much guaranteed that interested foreign parties would know about the shipment before it arrived at its destination.
With rail, it’s not like there are that many turnoffs, making it pretty easy to track the shipment.
With sea freight, unless there was a serious security breach, it would be extremely hard for foreign intelligence to work out which ship the missiles are on.
With China’s vast shipping infrastructure and the sheer volume of normal trade flow, even if foreign intelligence found out which ship was delivering the missiles, it would be incredibly easy for China to ‘loose’ those containers carrying the missiles in its commercial freight network and make it near impossible for outsiders to track where the missiles went after they landed.